Add end to end integration test for DevTools extensions #10699

Job	Run time
flutter-prep / ubuntu-latest Flutter Prep	32s
flutter-prep / macos-latest Flutter Prep	2m 12s
main	6m 46s
macos goldens test_dart2js	9m 3s
integration-test dart2js - flutter	34m 13s
integration-test dart2js - flutter-web	20m 12s
integration-test dart2js - dart-cli	18m 26s
build_ddc	3m 43s
build_dart2js	5m 37s
test_ddc	13m 48s
test_dart2js	13m 52s
	2h 8m 24s

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