Uncaught exceptions are printed twice in Firefox

[jamesderlin]

What happened?

In Firefox, uncaught exceptions are printed to the console twice and look confusing.

Steps to reproduce problem

Run:

void main() {
  throw Exception('hello');
}

In Firefox, the console shows:

: Exception: helloError: Exception: hello

This is fine in Chrome, however:

Uncaught Error: Exception: hello

Browser

Browser: Firefox 75.0 (64-bit)

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

No, I verified with a clean profile.

Are there any warnings or errors in your browser's JavaScript console? If so, paste them below:

: Exception: hello frame_dark.html line 38 > scriptElement:340:17
    wrapException https://dartpad.dartlang.org/scripts/frame_dark.html line 38 > scriptElement:340
    main https://dartpad.dartlang.org/scripts/frame_dark.html line 38 > scriptElement:923
    <anonymous> https://dartpad.dartlang.org/scripts/frame_dark.html line 38 > scriptElement:1183
    <anonymous> https://dartpad.dartlang.org/scripts/frame_dark.html line 38 > scriptElement:1167
    dartProgram https://dartpad.dartlang.org/scripts/frame_dark.html line 38 > scriptElement:1178
    <anonymous> https://dartpad.dartlang.org/scripts/frame_dark.html line 38 > scriptElement:1185
    replaceJavaScript https://dartpad.dartlang.org/scripts/frame_dark.html:38
    messageHandler https://dartpad.dartlang.org/scripts/frame_dark.html:54

Machine

Operating system: Linux