Fix a few strong mode errors in dart:html

tools/dom/templates/html/impl/impl_Element.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.
 
 part of $LIBRARYNAME;
 
 class _ChildrenElementList extends ListBase<Element>
     implements NodeListWrapper {
   // Raw Element.
   final Element _element;
@@ skipping 246 matching lines @@
   CssRect get marginEdge;
 $!STREAM_GETTER_SIGNATURES
 }
 
 // Wrapper over an immutable NodeList to make it implement ElementList.
 //
 // Clients are {`Document`, `DocumentFragment`}.`querySelectorAll` which are
 // declared to return `ElementList`.  This provides all the static analysis
 // benefit so there is no need for this class have a constrained type parameter.
 //
-class _FrozenElementList extends ListBase
-    implements ElementList, NodeListWrapper {
+class _FrozenElementList extends ListBase<Element>
+    implements ElementList<Element>, NodeListWrapper {
   final List<Node> _nodeList;
 
 $if JSINTEROP
   var dartClass_instance;
 
   _FrozenElementList._wrap(this._nodeList) {
       this.dartClass_instance = this._nodeList;
   }
 $else
   _FrozenElementList._wrap(this._nodeList);
@@ skipping 30 matching lines @@
   CssStyleDeclarationBase get style =>
       new _CssStyleDeclarationSet(this);
 
   set classes(Iterable<String> value) {
     // TODO(sra): This might be faster for Sets:
     //
     //     new _MultiElementCssClassSet(this).writeClasses(value)
     //
     // as the code below converts the Iterable[value] to a string multiple
     // times.  Maybe compute the string and set className here.
-    _nodeList.forEach((e) => e.classes = value);
+    forEach((e) => e.classes = value);
   }
 
   CssRect get contentEdge => new _ContentCssListRect(this);
 
   CssRect get paddingEdge => this.first.paddingEdge;
 
   CssRect get borderEdge => this.first.borderEdge;
 
   CssRect get marginEdge => this.first.marginEdge;
 
@@ skipping 996 matching lines @@
       throw new ArgumentError(
           'validator can only be passed if treeSanitizer is null');
     }
 
     if (_parseDocument == null) {
       _parseDocument = document.implementation.createHtmlDocument('');
       _parseRange = _parseDocument.createRange();
 
       // Workaround for Safari bug. Was also previously Chrome bug 229142
       // - URIs are not resolved in new doc.
-      var base = _parseDocument.createElement('base');
+      BaseElement base = _parseDocument.createElement('base');
       base.href = document.baseUri;
       _parseDocument.head.append(base);
     }
     var contextElement;
     if (this is BodyElement) {
       contextElement = _parseDocument.body;
     } else {
       contextElement = _parseDocument.createElement(tagName);
       _parseDocument.body.append(contextElement);
     }
@@ skipping 323 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }