Adding support checks for partially supported element types and updating tests.

sdk/lib/html/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 html;
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList implements List {
   // Raw Element.
@@ skipping 367 matching lines @@
    * [tag] should be a valid HTML tag name. If [tag] is an unknown tag then
    * this will create an [UnknownElement].
    *
    *     var divElement = new Element.tag('div');
    *     print(divElement is DivElement); // 'true'
    *     var myElement = new Element.tag('unknownTag');
    *     print(myElement is UnknownElement); // 'true'
    *
    * For standard elements it is more preferable to use the type constructors:
    *     var element = new DivElement();
+   *
+   * See also:
+   *
+   * * [isTagSupported]
    */
   factory $CLASSNAME.tag(String tag) =>
       _$(CLASSNAME)FactoryProvider.createElement_tag(tag);
 
   /**
    * All attributes on this element.
    *
    * Any modifications to the attribute map will automatically be applied to
    * this element.
    *
@@ skipping 193 matching lines @@
   }
 
   /**
    * Parses the specified text as HTML and adds the resulting node after the
    * last child of this element.
    */
   void appendHtml(String text) {
     this.insertAdjacentHtml('beforeend', text);
   }
 
+  /**
+   * Checks to see if the tag name is supported by the current platform.
+   *
+   * The tag should be a valid HTML tag name.
+   */
+  static bool isTagSupported(String tag) {
+    var e = _ElementFactoryProvider.createElement_tag(tag);
+    return e is Element && !(e is UnknownElement);
+  }
+
   // Hooks to support custom WebComponents.
   /**
    * Experimental support for [web components][wc]. This field stores a
    * reference to the component implementation. It was inspired by Mozilla's
    * [x-tags][] project. Please note: in the future it may be possible to
    * `extend Element` from your class, in which case this field will be
    * deprecated and will simply return this [Element] object.
    *
    * [wc]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html
    * [x-tags]: http://x-tags.org/
@@ skipping 176 matching lines @@
           'top level elements but 1 expected');
     }
     element.remove();
     return element;
   }
 
   /** @domName Document.createElement */
 $if DART2JS
   // Optimization to improve performance until the dart2js compiler inlines this
   // method.
-  static Element createElement_tag(String tag) =>
+  static dynamic createElement_tag(String tag) =>
       JS('Element', 'document.createElement(#)', tag);
 $else
   static Element createElement_tag(String tag) =>
       document.$dom_createElement(tag);
 $endif
 }