Add a conditional JS preinstall script instead of BAT file

gui/packages/yarn-fixes/package.json

@@ -3,7 +3,6 @@
   "name": "yarn-fixes",
   "version": "0.1.0",
   "scripts": {
-    "preinstall": "patch-yarn.bat"
-  },
-  "os": ["win32"]
+    "preinstall": "node ./patch-yarn.js"
+  }
 }

gui/packages/yarn-fixes/patch-yarn.js

@@ -0,0 +1,30 @@
+// Yarn 1.9.4 has a path lookup bug on Windows, when it looks for the binaries referenced in
+// scripts under '\gui\node_modules\node_modules' instead of '\gui\node_modules'.
+// This patch adds a junction between those two to keep that house of cards from falling apart.
+
+const path = require('path');
+const fs = require('fs');
+
+if (process.platform !== 'win32') {
+  return;
+}
+
+const sourcePath = path.resolve(path.join(__dirname, '../../node_modules'));
+const symlinkPath = path.join(__dirname, '../../node_modules/node_modules');
+
+try {
+  console.log('Removing a symlink to node_modules/node_modules');
+  fs.unlinkSync(symlinkPath);
+} catch (error) {
+  if (error.code !== 'ENOENT') {
+    throw error;
+  }
+}
+
+try {
+  console.log('Applying yarn workspaces patch for node_modules/node_modules');
+  fs.symlinkSync(sourcePath, symlinkPath, 'junction');
+  console.log('Done');
+} catch (error) {
+  console.error('Cannot symlink node_modules/node_modules: ' + error.message);
+}