Strip unused functionality from dart:html and fix strong mode errors. Switch from blacklisting pure…

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index 5f439a94087fb916b6ac5ed02536c8c8a5bcf966..5e4d69aca847eec66f76ea172b3f41abbeceb0ad 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -3353,7 +3353,7 @@ void checkDeferredIsLoaded(String loadId, String uri) {
  * objects that support integer indexing. This interface is not
  * visible to anyone, and is only injected into special libraries.
  */
-abstract class JavaScriptIndexingBehavior extends JSMutableIndexable {
+abstract class JavaScriptIndexingBehavior<E> extends JSMutableIndexable<E> {
 }
 
 // TODO(lrn): These exceptions should be implemented in core.