Convert legacy 'native code' to JS-forms in dart:html

lib/html/src/dart2js_MutationObserverSupported.dart

Index: lib/html/src/dart2js_MutationObserverSupported.dart
diff --git a/lib/html/src/dart2js_MutationObserverSupported.dart b/lib/html/src/dart2js_MutationObserverSupported.dart
index f7329073a696981376d34b9dbeadd0132765a700..2157df7a929db74b67ebfa928c0604db0e320df1 100644
--- a/lib/html/src/dart2js_MutationObserverSupported.dart
+++ b/lib/html/src/dart2js_MutationObserverSupported.dart
@@ -6,6 +6,5 @@
  * Checks to see if the mutation observer API is supported on the current
  * platform.
  */
-bool _isMutationObserverSupported() native '''
-  return !!( window.MutationObserver || window.WebKitMutationObserver);
-''';
+bool _isMutationObserverSupported() =>
+  JS('bool', '!!(window.MutationObserver || window.WebKitMutationObserver)');