Clean up error handling.

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

 // 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.
 
 library elements;
 
 
 import 'modelx.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
 import '../resolution/resolution.dart';
 
 import '../dart2jslib.dart' show InterfaceType,
                                  DartType,
                                  TypeVariableType,
                                  TypedefType,
+                                 DualKind,
                                  MessageKind,
                                  DiagnosticListener,
                                  Script,
                                  FunctionType,
                                  SourceString,
                                  Selector,
                                  Constant,
                                  Compiler;
 
 import '../dart_types.dart';
@@ skipping 545 matching lines @@
     return element == null || element.statement != body;
   }
 }
 
 abstract class ErroneousElement extends Element implements FunctionElement {
   MessageKind get messageKind;
   Map get messageArguments;
 }
 
 abstract class AmbiguousElement extends Element {
-  MessageKind get messageKind;
+  DualKind get messageKind;
   Map get messageArguments;
   Element get existingElement;
   Element get newElement;
 }
 
 // TODO(kasperl): This probably shouldn't be called an element. It's
 // just an interface shared by classes and libraries.
 abstract class ScopeContainerElement {
   Element localLookup(SourceString elementName);
 
@@ skipping 343 matching lines @@
   int get resolutionState;
   Token get beginToken;
   Token get endToken;
 
   // TODO(kasperl): Try to get rid of these.
   void set annotatedElement(Element value);
   void set resolutionState(int value);
 
   MetadataAnnotation ensureResolved(Compiler compiler);
 }