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

tools/dom/templates/html/impl/impl_Window.darttemplate

Index: tools/dom/templates/html/impl/impl_Window.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Window.darttemplate b/tools/dom/templates/html/impl/impl_Window.darttemplate
index c9f82796bb954f0b4c9e93e627a53c9e4de1d217..8abac3775c4146f65b923db522ae32ae4fa881ac 100644
--- a/tools/dom/templates/html/impl/impl_Window.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Window.darttemplate
@@ -339,8 +339,10 @@ class _BeforeUnloadEventStreamProvider implements
   const _BeforeUnloadEventStreamProvider(this._eventType);
 
   Stream<BeforeUnloadEvent> forTarget(EventTarget e, {bool useCapture: false}) {
-    var stream = new _EventStream(e, _eventType, useCapture);
 $if DART2JS
+    // Specify the generic type for EventStream only in dart2js to avoid
+    // checked mode errors in dartium.
+    var stream = new _EventStream<BeforeUnloadEvent>(e, _eventType, useCapture);
     var controller = new StreamController<BeforeUnloadEvent>(sync: true);
 
     stream.listen((event) {
@@ -350,6 +352,7 @@ $if DART2JS
 
     return controller.stream;
 $else
+    var stream = new _EventStream(e, _eventType, useCapture);
     return stream;
 $endif
   }
@@ -359,11 +362,23 @@ $endif
   }
 
   ElementStream<BeforeUnloadEvent> forElement(Element e, {bool useCapture: false}) {
+$if DART2JS
+    // Specify the generic type for _ElementEventStreamImpl only in dart2js to
+    // avoid checked mode errors in dartium.
+    return new _ElementEventStreamImpl<BeforeUnloadEvent>(e, _eventType, useCapture);
+$else
     return new _ElementEventStreamImpl(e, _eventType, useCapture);
+$endif
   }
 
   ElementStream<BeforeUnloadEvent> _forElementList(ElementList e,
       {bool useCapture: false}) {
+$if DART2JS
+    // Specify the generic type for _ElementEventStreamImpl only in dart2js to
+    // avoid checked mode errors in dartium.
+    return new _ElementListEventStreamImpl<BeforeUnloadEvent>(e, _eventType, useCapture);
+$else
     return new _ElementListEventStreamImpl(e, _eventType, useCapture);
+$endif
   }
 }