Reapply "Fix analyzer errors in html Dartium."

tools/dom/src/dartium_CustomElementSupport.dart

 // Copyright (c) 2014, 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 dart.dom.html;
 
 /// Dartium ElementUpgrader implementation.
 class _VMElementUpgrader implements ElementUpgrader {
   final Type _type;
   final Type _nativeType;
@@ skipping 12 matching lines @@
         throw new UnsupportedError(
             'extendsTag does not match base native class');
       }
     }
   }
 
   Element upgrade(Element element) {
     if (element.runtimeType != _nativeType) {
       throw new UnsupportedError('Element is incorrect type');
     }
-    return _Utils.changeElementWrapper(element, _type);
+    _Utils.changeElementWrapper(element, _type);
+    return null;
   }
 }
 
 /// Validates that the custom type is properly formed-
 ///
 /// * Is a user-defined class.
 /// * Has a created constructor with zero args.
 /// * Derives from an Element subclass.
 ///
 /// Then returns the native base class.
@@ skipping 35 matching lines @@
   return nativeClass;
 }
 
 
 bool _isBuiltinType(ClassMirror cls) {
   // TODO(vsm): Find a less hackish way to do this.
   LibraryMirror lib = cls.owner;
   String libName = lib.uri.toString();
   return libName.startsWith('dart:');
 }