nit: make unreachable statement a warning

Signed-off-by: Jean Privat <[email protected]>

src/semantize/flow.nit

@@ -305,7 +305,7 @@ redef class ABlockExpr
 				v.enter_visit(e)
 			else if not v.current_flow_context.is_already_unreachable then
 				v.current_flow_context.is_already_unreachable = true
-				v.toolcontext.error(e.hot_location, "Error: unreachable statement.")
+				v.toolcontext.warning(e.hot_location, "unreachable", "Warning: unreachable statement.")
 			end
 		end
 	end

src/semantize/local_var_init.nit

@@ -59,17 +59,17 @@ private class LocalVarInitVisitor
 		assert variable != null
 		if not maybe_unset_vars.has(variable) then return
 
-		var flow = node.after_flow_context.as(not null)
-		flow.set_vars.add(variable)
+		var flow = node.after_flow_context
+		if flow != null then flow.set_vars.add(variable)
 	end
 
 	fun check_is_set(node: AExpr, variable: nullable Variable)
 	do
 		assert variable != null
 		if not maybe_unset_vars.has(variable) then return
 
-		var flow = node.after_flow_context.as(not null)
-		if not flow.is_variable_set(variable) then
+		var flow = node.after_flow_context
+		if flow != null and not flow.is_variable_set(variable) then
 			self.toolcontext.error(node.hot_location, "Error: possibly unset variable `{variable}`.")
 			# Remove the variable to avoid repeating errors
 			self.maybe_unset_vars.remove(variable)

src/semantize/typing.nit

@@ -634,9 +634,7 @@ private class TypeVisitor
 	fun set_variable(node: AExpr, variable: Variable, mtype: nullable MType)
 	do
 		var flow = node.after_flow_context
-		assert flow != null
-
-		flow.set_var(self, variable, mtype)
+		if flow != null then flow.set_var(self, variable, mtype)
 	end
 
 	# Find the exact representable most specific common super-type in `col`.