assert() shows opaque error

[johnpryan]

When an assert() statement fails, "Script error." appears in the console.

We should investigate if assert statements should be enabled in DartPad, and how to display a more helpful error.

Related issues:

• Assertion errors print poorly #1580
• "Script error." - No line number indicated #1649
• Using Colors.black as a primarySwatch causes an opaque error #1650
• dartpad.dev - "Script Error" with bunch of javascript errors in the chrome console. #1925
• Script error #2514

[johnpryan]

Here's an updated example - this was filed before the framework was updated to null safety, so most of the asserts in the framework can't be hit anymore.

[johnpryan]

Filed flutter/flutter#92191 to track unnecessary asserts in the SDK

[nshahan]

I can see the assertion failure message appear in the browser DevTools console. Is this just a matter of surfacing those messages in the DartPad console?

[johnpryan]

Yes that would help. We should probably show the red error widget too. I'm not sure if dart-archive/dart-services#737 would help with that, or if the issue is just how the frontend is handling the error.

[johnpryan]

Gathered a bit more on this: the assertion failure in the example should be one of the VisualDensity constructor's asserts, but the assertion that triggers is:

errors.dart:202 Uncaught Error: Assertion failed: file:///Users/brettmorgan/Documents/GitHub/dart-services/flutter-sdk/packages/flutter/lib/src/widgets/framework.dart:2577:20
_debugCurrentBuildTarget == context
is not true
    at Object.throw_ [as throw] (errors.dart:251)
    at Object.assertFailed (errors.dart:29)
    at framework.dart:2577
    at framework.BuildOwner.new.buildScope (framework.dart:2580)
    at RenderObjectToWidgetAdapter.new.attachToRenderTree (binding.dart:1111)
    at binding$.WidgetsFlutterBinding.new.attachRootWidget (binding.dart:943)
    at binding.dart:924
    at internalCallback (isolate_helper.dart:48)

Here's the line where the assert is defined: https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/widgets/framework.dart#L2535

This is happening on the dart-services master branch and dart-archive/dart-services#737.

[parlough]

Consolidating issue in to #3008 alongside other error surfacing improvements. Thanks for the discussion and investigation here!