Fix type annotations that violate subtyping constraint on overriden methods.

pkg/compiler/lib/src/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 '../constants/expressions.dart';
 import '../helpers/helpers.dart';  // Included for debug helpers.
 import '../tree/tree.dart';
@@ skipping 1341 matching lines @@
   LocalVariableElementX(String name,
                         ExecutableElement enclosingElement,
                         VariableList variables,
                         Token token)
       : super(name, ElementKind.VARIABLE, enclosingElement, variables, token) {
     createDefinitions(variables.definitions);
   }
 
   ExecutableElement get executableContext => enclosingElement;
 
-  ExecutableElement get memberContext => executableContext.memberContext;
+  MemberElement get memberContext => executableContext.memberContext;
 
   bool get isLocal => true;
 }
 
 class FieldElementX extends VariableElementX
     with AnalyzableElementX implements FieldElement {
   List<FunctionElement> nestedClosures = new List<FunctionElement>();
 
   FieldElementX(Identifier name,
                 Element enclosingElement,
@@ skipping 1690 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }