Refactor and simplify the dart2dart renamer.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

 // Copyright (c) 2013, 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.
 
 library elements.modelx;
 
 import 'elements.dart';
 import '../helpers/helpers.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 281 matching lines @@
   get executableContext => unsupported();
 
   bool get isRedirectingFactory => unsupported();
 
   computeSignature(compiler) => unsupported();
 
   get effectiveTarget => this;
 
   computeEffectiveTargetType(InterfaceType newType) => unsupported();
 
-  get definingConstructor => this;
+  get definingConstructor => null;
 
   FunctionElement asFunctionElement() => this;
 
   String get message => '${messageKind.message(messageArguments)}';
 
   String toString() => '<$name: $message>';
 
   accept(ElementVisitor visitor) => visitor.visitErroneousElement(this);
 }
 
@@ skipping 2343 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }