Local DartPad UI unable to reach local backend

[craiglabenz]

What happened?

I tried to run DartPad locally to test the addition of a new dependency, but the frontend (in Chrome) is unable to reach the backend on account of a security error.

Steps to reproduce problem

Terminal 1

$ cd pkgs/dart_services && dart bin/server.dart 

Terminal 2

$ cd pkgs/dartpad_ui && * flutter run -d chrome --web-browser-flag "--disable-web-security"

Then in Chrome once it launches, I clicked > Run and saw this error:

rtti.dart:114 Uncaught (in promise) SecurityError: Blocked a frame with origin "http://localhost:60142" from accessing a cross-origin frame.
    at hasOwnProperty (<anonymous>)
    at Object.getInterceptorForRti (rtti.dart:114:37)
    at dart_rti.Rti.new._isTestViaProperty (rti.dart:1332:21)
    at dart_rti.Rti.new._generalAsCheckImplementation (rti.dart:1382:34)
    at Object.JSObjectUnsafeUtilExtension$124getProperty [as JSObjectUnsafeUtilExtension|getProperty] (js_interop_unsafe_patch.dart:20:7)
    at Object.HTMLIFrameElementExtension$124safelyPostMessage [as HTMLIFrameElementExtension|safelyPostMessage] (frame_utils.dart:27:10)
    at [_send] (frame.dart:142:12)
    at frame.dart:35:12
    at async_patch.dart:603:19
    at async_patch.dart:627:23
    at async_patch.dart:574:31
    at _RootZone.runUnary (zone.dart:1661:54)
    at async._FutureListener.thenAwait.handleValue (future_impl.dart:163:18)
    at handleValueCallback (future_impl.dart:861:44)
    at _Future._propagateToListeners (future_impl.dart:890:13)
    at [_completeWithValue] (future_impl.dart:666:5)
    at future_impl.dart:966:16
    at _RootZone.runUnary (zone.dart:1661:54)
    at async._FutureListener.then.handleValue (future_impl.dart:163:18)
    at handleValueCallback (future_impl.dart:861:44)
    at _Future._propagateToListeners (future_impl.dart:890:13)
    at [_completeWithValue] (future_impl.dart:666:5)
    at async._AsyncCallbackEntry.new.callback (future_impl.dart:736:7)
    at Object._microtaskLoop (schedule_microtask.dart:40:11)
    at _startMicrotaskLoop (schedule_microtask.dart:49:5)
    at async_patch.dart:179:7

Additional info

Browser

Browser: Chrome

Version: 128.0.6613.120 (Official Build) (arm64)

Are you using any extensions/plugins that affect website behavior
(particularly those that affect iframes, such as ad blockers)?

No.

Machine

Operating system: macOS

Version: Sonoma 14.6.1 (23G93)

Your code

I encountered this issue while attempting to manually test #3053

DartPad's output

See error above.

[johnpryan]

cc: @devoncarew @parlough