dartanalyzer (internal testing tool) --json does not respect diagnostic suppression

[srawlins]

If I place a analysis_options.yaml file in the root of the Dart SDK repo like this:

analyzer:
  errors:
    deprecated_subtype_of_function: ignore
    unused_import: ignore
    unused_local_variable: ignore

and build the dartanalyzer tool, which is used by tools/test.py, and run this command, I see errors which should be suppressed:

$ DART_CONFIGURATION=ReleaseX64 xcodebuild/ReleaseX64/dart-sdk/bin/dart xcodebuild/ReleaseX64/gen/dartanalyzer.dart.snapshot -Dtest_runner.configuration=analyzer-mac --ignore-unrecognized-flags --packages=/Users/srawlins/code/dart-sdk/sdk/.dart_tool/package_config.json --format=json   /Users/srawlins/code/dart-sdk/sdk/xcodebuild/ReleaseX64/generated_tests/language_2/call/method_as_cast_test_01.dart

{"version":1,"diagnostics":[{"code":"deprecated_subtype_of_function","severity":"WARNING","type":"STATIC_WARNING","location":{"file":"/Users/srawlins/code/dart-sdk/sdk/xcodebuild/ReleaseX64/generated_tests/language_2/call/method_as_cast_test_01.dart","range":{"start":{"offset":387,"line":16,"column":20},"end":{"offset":395,"line":16,"column":28}}},"problemMessage":"Implementing 'Function' has no effect.","correctionMessage":"Try removing 'Function' from the 'implements' clause.","documentation":"https://dart.dev/diagnostics/deprecated_subtype_of_function"},{"code":"unused_local_variable","severity":"INFO","type":"HINT","location":{"file":"/Users/srawlins/code/dart-sdk/sdk/xcodebuild/ReleaseX64/generated_tests/language_2/call/method_as_cast_test_01.dart","range":{"start":{"offset":756,"line":33,"column":5},"end":{"offset":757,"line":33,"column":6}}},"problemMessage":"The value of the local variable 'd' isn't used.","correctionMessage":"Try removing the variable or using it.","documentation":"https://dart.dev/diagnostics/unused_local_variable"}]}

But if I remove the --format=json, we see the errors correctly suppressed:

$ DART_CONFIGURATION=ReleaseX64 xcodebuild/ReleaseX64/dart-sdk/bin/dart xcodebuild/ReleaseX64/gen/dartanalyzer.dart.snapshot -Dtest_runner.configuration=analyzer-mac --ignore-unrecognized-flags --packages=/Users/srawlins/code/dart-sdk/sdk/.dart_tool/package_config.json /Users/srawlins/code/dart-sdk/sdk/xcodebuild/ReleaseX64/generated_tests/language_2/call/method_as_cast_test_01.dart
Analyzing /Users/srawlins/code/dart-sdk/sdk/xcodebuild/ReleaseX64/generated_tests/language_2/call/method_as_cast_test_01.dart...
No issues found!

[srawlins]

p1 because this is blocking the migration away from Hints.