Add `responseHeaders` extension on `XMLHttpRequest` #28

Job	Run time
mono_repo self validate	18s
analyze_and_format; Dart 3.4.0; PKG: web; `dart analyze --fatal-infos .`	23s
analyze_and_format; Dart 3.4.0; PKG: web; `dart format --output=none --set-exit-if-changed .`	18s
analyze_and_format; Dart dev; PKGS: web, web_generator; `dart analyze --fatal-infos .`	32s
analyze_and_format; Dart dev; PKGS: web, web_generator; `dart format --output=none --set-exit-if-...	25s
unit_test; Dart dev; PKGS: web, web_generator; `dart test --test-randomize-ordering-seed=random -...	54s
unit_test; Dart 3.4.0; PKG: web; `dart test --test-randomize-ordering-seed=random -p chrome -c da...	31s
unit_test; Dart 3.4.0; PKG: web; `dart test --test-randomize-ordering-seed=random -p chrome`	1m 13s
unit_test; Dart dev; PKGS: web, web_generator; `dart test --test-randomize-ordering-seed=random -...	50s
generate_and_analyze; Dart dev; PKG: web_generator; `dart analyze --fatal-infos .`	24s
generate_and_analyze; Dart dev; PKG: web_generator; `dart bin/update_bindings.dart`	47s
dart_fixes; Dart main; PKG: web; `dart fix --compare-to-golden test_fixes`	19s
	6m 54s

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