Add function to set structured error early

[helin24]

Description

We noticed while testing exception logging in flutter-intellij that early exceptions (e.g. we tested throwing an exception immediately in the build method of GalleryApp) aren't handled by the structured handler. This change adds an option for a project to use the structured handler immediately instead of waiting for it to be registered later.

Run output before change:

Running "flutter pub get" in gallery...                             0.7s
Launching lib/main.dart on iPhone SE (2nd generation) in debug mode...
Running pod install...                                              2.4s
Running Xcode build...
Xcode build done.                                           53.2s
Debug service listening on ws://127.0.0.1:52394/VOAW1GUS21Y=/ws
Syncing files to device iPhone SE (2nd generation)...
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following FormatException was thrown building GalleryApp(dirty):
flutter: FormatException
flutter:
flutter: The relevant error-causing widget was:
flutter:   GalleryApp file:///Users/helinx/Documents/gallery/lib/main.dart:20:16
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0      GalleryApp.build (package:gallery/main.dart:35:5)
flutter: #1      StatelessElement.build (package:flutter/src/widgets/framework.dart:4580:28)
flutter: #2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4506:15)
flutter: #3      Element.rebuild (package:flutter/src/widgets/framework.dart:4222:5)
flutter: #4      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4485:5)
flutter: #5      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4480:5)
flutter: #6      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3450:14)
flutter: #7      Element.updateChild (package:flutter/src/widgets/framework.dart:3218:18)
flutter: #8      RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1130:16)
flutter: #9      RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1101:5)
flutter: #10     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1043:17)
flutter: #11     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2607:19)
flutter: #12     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1042:13)
flutter: #13     WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:923:7)
flutter: #14     WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:904:7)
flutter: (elided 11 frames from class _RawReceivePortImpl, class _Timer, dart:async, and dart:async-patch)
flutter:
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following FormatException was thrown building GalleryApp(dirty):
FormatException

The relevant error-causing widget was: 
  GalleryApp file:///Users/helinx/Documents/gallery/lib/main.dart:20:16
When the exception was thrown, this was the stack: 
#0      GalleryApp.build (package:gallery/main.dart:35:5)
#1      StatelessElement.build (package:flutter/src/widgets/framework.dart:4580:28)
#2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4506:15)
#3      Element.rebuild (package:flutter/src/widgets/framework.dart:4222:5)
#4      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2627:33)
...
════════════════════════════════════════════════════════════════════════════════════════════════════

Run output after change:

Launching lib/main.dart on iPhone SE (2nd generation) in debug mode...
Running Xcode build...
Xcode build done.                                           33.8s
Waiting for iPhone SE (2nd generation) to report its views...
Debug service listening on ws://127.0.0.1:55771/43D3tM6g8xM=/ws
Syncing files to device iPhone SE (2nd generation)...

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following FormatException was thrown building GalleryApp(dirty):
FormatException

The relevant error-causing widget was: 
  GalleryApp file:///Users/helinx/Documents/gallery/lib/main.dart:22:16
When the exception was thrown, this was the stack: 
#0      GalleryApp.build (package:gallery/main.dart:37:5)
#1      StatelessElement.build (package:flutter/src/widgets/framework.dart:4585:28)
#2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4511:15)
#3      Element.rebuild (package:flutter/src/widgets/framework.dart:4227:5)
#4      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2632:33)
...
════════════════════════════════════════════════════════════════════════════════════════════════════

Related Issues

(None - should I file one?)

Tests

I added the following tests:

• Check that error events are received after setStructuredErrors has been called

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
  • I wrote a design doc: https://flutter.dev/go/template Replace this with a link to your design doc's short link
  • I got input from the developer relations team, specifically from: Replace with the names of who gave advice
  • I wrote a migration guide: Replace with a link to your migration guide

cc @jacob314

[jacob314]

lgtm with a couple nits.

[jonahwilliams]

LGTM