Add supported checks to the SVG library, and library cleanup.

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

 // Copyright (c) 2011, 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 _AttributeClassSet extends CssClassSet {
   final Element _element;
 
   _AttributeClassSet(this._element);
@@ skipping 93 matching lines @@
 
   HtmlCollection get $dom_children {
     throw new UnsupportedError("Cannot get dom_children on SVG.");
   }
 
   bool get isContentEditable => false;
   void click() {
     throw new UnsupportedError("Cannot invoke click SVG.");
   }
 
+  /**
+   * Checks to see if the SVG element type is supported by the current platform.
+   *
+   * The tag should be a valid SVG element tag name.
+   */
+  static bool isTagSupported(String tag) {
+    var e = new $CLASSNAME.tag(tag);
+    return e is $CLASSNAME && !(e is UnknownElement);
+  }
 $!MEMBERS
 }