Make new StreamController be async by default.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 @Experimental
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
@@ skipping 11 matching lines @@
    * should not be used in your code.
    *
    * This event is fired whenever a template is instantiated via
    * [createInstance].
    */
   // TODO(rafaelw): This is a hack, and is neccesary for the polyfill
   // because custom elements are not upgraded during clone()
   @Experimental
   static Stream<DocumentFragment> get instanceCreated {
     if (_instanceCreated == null) {
-      _instanceCreated = new StreamController<DocumentFragment>();
+      _instanceCreated = new StreamController<DocumentFragment>(sync: true);
     }
     return _instanceCreated.stream;
   }
 
   /**
    * Ensures proper API and content model for template elements.
    *
    * [instanceRef] can be used to set the [Element.ref] property of [template],
    * and use the ref's content will be used as source when createInstance() is
    * invoked.
@@ skipping 83 matching lines @@
     document.head.append(style);
   }
 
   /**
    * A mapping of names to Custom Syntax objects. See [CustomBindingSyntax] for
    * more information.
    */
   @Experimental
   static Map<String, CustomBindingSyntax> syntax = {};
 }