A Dart interface for GoJS: https://gojs.net/

A description of the library from gojs.net:

"GoJS is a JavaScript and TypeScript library for building interactive diagrams and graphs. GoJS allows you to build all kinds of diagrams and graphs for your users, from simple flowcharts and org charts to highly-specific industrial diagrams, SCADA and BPMN diagrams, medical diagrams like genograms, and more. GoJS makes constructing JavaScript diagrams of complex nodes, links, and groups easy with customizable templates and layouts.

GoJS offers many advanced features for user interactivity such as drag-and-drop, copy-and-paste, in-place text editing, tooltips, context menus, automatic layouts, templates, data binding and models, transactional state and undo management, palettes, overviews, event handlers, commands, extensible tools for custom operations, and customizable animations."

You can checkout for working samples of the library here: https://gojs.net/latest/samples/index.html

Simple example:

import 'package:gojs/gojs.dart';
import 'dart:html' as html;

// some simple html with a div like <div id="diagram"></div>

void main() {
  var diagram = html.document.getElementById('diagram');
  
  var myDiagram = GoJSDiagram(diagram)
    ..model = GoJSModel.fromJson('''
        [
          { "key": "Alpha" },
          { "key": "Beta" },
          { "key": "Gamma" }
        ]
    ''');
}

For a full working flowchart app, checkout for example/flowchart.

import 'package:gojs/gojs.dart';
import 'package:js/js.dart';
import 'dart:html' as html;

void main() {
  var diagram = html.document.getElementById('diagram');
  var palette = html.document.getElementById('palette');

  try {
    var god = GoJSDiagram(diagram)
      ..linkDrawn = allowInterop(showLinkLabel)
      ..linkRelinked = allowInterop(showLinkLabel);

    god.nodeTemplateMap.add(
        '', // the default category
        nodeStyle()
          ..type = GoJSPanel.table
          ..addAll([
            GoJSPanel()
              ..type = GoJSPanel.auto
              ..addAll([
                GoJSShape()
                  ..figure = 'Rectangle'
                  ..fill = '#282c34'
                  ..stroke = '#00A9C9'
                  ..strokeWidth = 3.5
                  ..bind(GoJSBinding('figure', 'figure')),
                textStyle()
                  ..margin = 8
                  ..maxSize = GoJSSize(160, nan)
                  ..wrap = GoJSTextBlock.wrapFit
                  ..editable = true
                  ..bind(GoJSBinding('text').makeTwoWay())
              ]),
            makePort('T', GoJSSpot.top, GoJSSpot.topSide, false, true),
            makePort('L', GoJSSpot.left, GoJSSpot.leftSide, true, true),
            makePort('R', GoJSSpot.right, GoJSSpot.rightSide, true, true),
            makePort('B', GoJSSpot.bottom, GoJSSpot.bottomSide, true, false)
          ]));

    god.nodeTemplateMap.add(
        'Conditional',
        nodeStyle()
          ..type = GoJSPanel.table
          ..addAll([
            GoJSPanel()
              ..type = GoJSPanel.auto
              ..addAll([
                GoJSShape()
                  ..figure = 'Diamond'
                  ..fill = '#282c34'
                  ..stroke = '#00A9C9'
                  ..strokeWidth = 3.5
                  ..bind(GoJSBinding('figure', 'figure')),
                textStyle()
                  ..margin = 8
                  ..maxSize = GoJSSize(160, nan)
                  ..wrap = GoJSTextBlock.wrapFit
                  ..editable = true
                  ..bind(GoJSBinding('text').makeTwoWay())
              ]),
            makePort('T', GoJSSpot.top, GoJSSpot.top, false, true),
            makePort('L', GoJSSpot.left, GoJSSpot.left, true, true),
            makePort('R', GoJSSpot.right, GoJSSpot.right, true, true),
            makePort('B', GoJSSpot.bottom, GoJSSpot.bottom, true, false)
          ]));

    god.nodeTemplateMap.add(
        'Start',
        nodeStyle()
          ..type = GoJSPanel.table
          ..addAll([
            GoJSPanel()
              ..type = GoJSPanel.auto
              ..addAll([
                GoJSShape()
                  ..figure = 'Circle'
                  ..fill = '#282c34'
                  ..stroke = '#09d3ac'
                  ..strokeWidth = 3.5
                  ..desiredSize = GoJSSize(70, 70),
                textStyle('Start')..bind(GoJSBinding('text'))
              ]),
            makePort('L', GoJSSpot.left, GoJSSpot.left, true, false),
            makePort('R', GoJSSpot.right, GoJSSpot.right, true, false),
            makePort('B', GoJSSpot.bottom, GoJSSpot.bottom, true, false)
          ]));

    god.nodeTemplateMap.add(
        'End',
        nodeStyle()
          ..type = GoJSPanel.table
          ..addAll([
            (GoJSPanel()
              ..type = GoJSPanel.auto
              ..addAll([
                GoJSShape()
                  ..figure = 'Circle'
                  ..fill = '#282c34'
                  ..stroke = '#DC3C00'
                  ..strokeWidth = 3.5
                  ..desiredSize = GoJSSize(70, 70),
                textStyle('End')..bind(GoJSBinding('text'))
              ])),
            makePort('T', GoJSSpot.top, GoJSSpot.top, false, true),
            makePort('L', GoJSSpot.left, GoJSSpot.left, false, true),
            makePort('R', GoJSSpot.right, GoJSSpot.right, false, true)
          ]));

    GoJSShape.defineFigureGenerator('File', allowInterop((shape, w, h) {
      var geo = GoJSGeometry();
      var fig = GoJSPathFigure(0, 0, true); // starting point
      geo.add(fig);
      fig.addAll([
        GoJSPathSegment(GoJSPathSegment.line, .75 * w, 0),
        GoJSPathSegment(GoJSPathSegment.line, w, .25 * h),
        GoJSPathSegment(GoJSPathSegment.line, w, h),
        GoJSPathSegment(GoJSPathSegment.line, 0, h).close()
      ]);
      var fig2 = GoJSPathFigure(.75 * w, 0, false);
      geo.add(fig2);
      // The Fold
      fig2.addAll([
        GoJSPathSegment(GoJSPathSegment.line, .75 * w, .25 * h),
        GoJSPathSegment(GoJSPathSegment.line, w, .25 * h)
      ]);
      geo
        ..spot1 = GoJSSpot(0, .25)
        ..spot2 = GoJSSpot.bottomRight;
      return geo;
    }));

    god.nodeTemplateMap.add(
        'Comment',
        nodeStyle()
          ..type = GoJSPanel.auto
          ..addAll([
            GoJSShape()
              ..figure = 'File'
              ..fill = '#282c34'
              ..stroke = '#DEE0A3'
              ..strokeWidth = 3,
            textStyle()
              ..margin = 8
              ..maxSize = GoJSSize(200, nan)
              ..wrap = GoJSTextBlock.wrapFit
              ..textAlign = 'center'
              ..editable = true
              ..bind(GoJSBinding('text').makeTwoWay())
          ]));

    god.linkTemplate = GoJSLink()
      ..routing = GoJSLink.avoidsNodes
      ..curve = GoJSLink.jumpOver
      ..corner = 5
      ..toShortLength = 4
      ..relinkableFrom = true
      ..relinkableTo = true
      ..reshapable = true
      ..resegmentable = true
      // mouse-overs subtly highlight links:
      ..mouseEnter = allowInterop(([e, link, k]) {
        (link.findObject('HIGHLIGHT') as GoJSShape).stroke =
            'rgba(30,144,255,0.2)';
      })
      ..mouseLeave = allowInterop(([e, link, k]) {
        (link.findObject('HIGHLIGHT') as GoJSShape).stroke = 'transparent';
      })
      ..selectionAdorned = false
      ..bind(GoJSBinding('points').makeTwoWay())
      ..addAll([
        GoJSShape() // the highlight shape, normally transparent
          ..isPanelMain = true
          ..strokeWidth = 8
          ..stroke = 'transparent'
          ..name = 'HIGHLIGHT',
        GoJSShape() // the link path shape
          ..isPanelMain = true
          ..strokeWidth = 2
          ..stroke = 'gray'
          ..bind(GoJSBinding('stroke', 'isSelected', allowInterop((sel, k) {
            return sel ? 'dodgerblue' : 'gray';
          })).ofObject()),
        GoJSShape() // the arrowhead
          ..toArrow = 'standard'
          ..strokeWidth = 0
          ..fill = 'gray',
        GoJSPanel()
          ..type = GoJSPanel.auto
          ..visible = false
          ..name = 'LABEL'
          ..segmentIndex = 2
          ..segmentFraction = 0.5
          ..bind(GoJSBinding('visible', 'visible').makeTwoWay())
          ..addAll([
            GoJSShape()
              ..figure = 'RoundedRectangle' // the label shape
              ..fill = '#F8F8F8'
              ..strokeWidth = 0,
            GoJSTextBlock()
              ..text = 'Yes'
              ..textAlign = 'center'
              ..font = '10pt helvetica, arial, sans-serif'
              ..stroke = '#333333'
              ..editable = true
              ..bind(GoJSBinding('text').makeTwoWay())
          ])
      ]);

    god.toolManager.linkingTool.temporaryLink.routing = GoJSLink.orthogonal;
    god.toolManager.relinkingTool.temporaryLink.routing = GoJSLink.orthogonal;

    // load
    var load = '''
      { "class": "go.GraphLinksModel",
        "linkFromPortIdProperty": "fromPort",
        "linkToPortIdProperty": "toPort",
        "nodeDataArray": [
      {"category":"Comment", "loc":"360 -10", "text":"Kookie Brittle", "key":-13},
      {"key":-1, "category":"Start", "loc":"175 0", "text":"Start"},
      {"key":0, "loc":"-5 75", "text":"Preheat oven to 375 F"},
      {"key":1, "loc":"175 100", "text":"In a bowl, blend: 1 cup margarine, 1.5 teaspoon vanilla, 1 teaspoon salt"},
      {"key":2, "loc":"175 200", "text":"Gradually beat in 1 cup sugar and 2 cups sifted flour"},
      {"key":3, "loc":"175 290", "text":"Mix in 6 oz (1 cup) Nestle's Semi-Sweet Chocolate Morsels"},
      {"key":4, "loc":"175 380", "text":"Press evenly into ungreased 15x10x1 pan"},
      {"key":5, "loc":"355 85", "text":"Finely chop 1/2 cup of your choice of nuts"},
      {"key":6, "loc":"175 450", "text":"Sprinkle nuts on top"},
      {"key":7, "loc":"175 515", "text":"Bake for 25 minutes and let cool"},
      {"key":8, "loc":"175 585", "text":"Cut into rectangular grid"},
      {"key":-2, "category":"End", "loc":"175 660", "text":"Enjoy!"}
      ],
        "linkDataArray": [
      {"from":1, "to":2, "fromPort":"B", "toPort":"T"},
      {"from":2, "to":3, "fromPort":"B", "toPort":"T"},
      {"from":3, "to":4, "fromPort":"B", "toPort":"T"},
      {"from":4, "to":6, "fromPort":"B", "toPort":"T"},
      {"from":6, "to":7, "fromPort":"B", "toPort":"T"},
      {"from":7, "to":8, "fromPort":"B", "toPort":"T"},
      {"from":8, "to":-2, "fromPort":"B", "toPort":"T"},
      {"from":-1, "to":0, "fromPort":"B", "toPort":"T"},
      {"from":-1, "to":1, "fromPort":"B", "toPort":"T"},
      {"from":-1, "to":5, "fromPort":"B", "toPort":"T"},
      {"from":5, "to":4, "fromPort":"B", "toPort":"T"},
      {"from":0, "to":4, "fromPort":"B", "toPort":"T"}
      ]}
      ''';

    god.model = GoJSModel.fromJson(load);

    var buf = '''
              { "class": "go.GraphLinksModel",
        "nodeDataArray": [{"category": "Start", "text": "Start"},
              {"text": "Step"},
              {"category": "Conditional", "text": "???"},
              {"category": "End", "text": "End"},
              {"category": "Comment", "text": "Comment"}]
              }
          ''';
    GoJSPalette(palette)
      ..nodeTemplateMap = god.nodeTemplateMap
      //..model = GoJSLinksModel(lm)
      ..model = GoJSModel.fromJson(buf)
      ..initialAnimationStarting = allowInterop(animateFadeDown)
      ..animationManager.initialAnimationStyle = GoJSAnimationManager.none;
  } catch (e) {
    print('FlowChart example error: $e');
    html.window.console.error(e);
  } finally {
    print('Done! :D');
  }
}

void animateFadeDown(e) {
  var diagram = e.diagram;
  var animation = GoJSAnimation();
  animation.isViewportUnconstrained =
      true; // So Diagram positioning rules let the animation start off-screen
  animation.easing = GoJSAnimation.easeOutExpo;
  animation.duration = 900;
  // Fade "down", in other words, fade in from above
  animation.add(diagram, 'position', diagram.position.copy().offset(0, 200),
      diagram.position);
  animation.add(diagram, 'opacity', 0, 1);
  animation.start();
}

void showLinkLabel(e) {
  var label = e.subject.findObject('LABEL');

  if (label == null) {
    label.visible = (e.subject.fromNode.data.category == 'Conditional');
  }
}

GoJSNode nodeStyle() => GoJSNode()
  ..locationSpot = GoJSSpot.center
  ..bind(GoJSBinding('location', 'loc', GoJSPoint.parse)
      .makeTwoWay(GoJSPoint.stringify));

GoJSTextBlock textStyle([String text]) => GoJSTextBlock()
  ..font = 'bold 11pt Lato, Helvetica, Arial, sans-serif'
  ..stroke = '#F8F8F8'
  ..text = text ?? undefined;

GoJSShape makePort(
    String name, GoJSSpot align, GoJSSpot spot, dynamic output, dynamic input) {
  var horizontal = align.equals(GoJSSpot.top) || align.equals(GoJSSpot.bottom);
  // the port is basically just a transparent rectangle that stretches along the side of the node,
  // and becomes colored when the mouse passes over it
  return GoJSShape()
    ..fill = 'transparent' // changed to a color in the mouseEnter event handler
    ..strokeWidth = 0 // no stroke
    ..width =
        horizontal ? nan : 8 // if not stretching horizontally, just 8 wide
    ..height =
        !horizontal ? nan : 8 // if not stretching vertically, just 8 tall
    ..alignment = align // align the port on the main Shape
    ..stretch =
        (horizontal ? GoJSGraphObject.horizontal : GoJSGraphObject.vertical)
    ..portId = name // declare this object to be a "port"
    ..fromSpot = spot // declare where links may connect at this port
    ..fromLinkable = output // declare whether the user may draw links from here
    ..toSpot = spot // declare where links may connect at this port
    ..toLinkable = input // declare whether the user may draw links to here
    ..cursor =
        'pointer' // show a different cursor to indicate potential link point
    ..mouseEnter = allowInterop(([e, port, k]) {
      // the PORT argument will be this Shape
      if (!e.diagram.isReadOnly) port.fill = 'rgba(255,0,255,0.5)';
    })
    ..mouseLeave = allowInterop(([e, port, k]) {
      port.fill = 'transparent';
    });
}