Make addGetterOrSetter (now: defineAccessor) not add the abstract field as a member.

lib/compiler/implementation/resolver.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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   Type getType(TypeAnnotation annotation);
 }
 
@@ skipping 261 matching lines @@
 
   void checkMembers(ClassElement cls) {
     if (cls === compiler.objectClass) return;
     cls.forEachMember((holder, member) {
       checkAbstractField(member);
       checkValidOverride(member, cls.lookupSuperMember(member.name));
     });
   }
 
   void checkAbstractField(Element member) {
-    if (member is !AbstractFieldElement) return;
-    if (member.getter === null) return;
-    if (member.setter === null) return;
-    int getterFlags = member.getter.modifiers.flags | Modifiers.FLAG_ABSTRACT;
-    int setterFlags = member.setter.modifiers.flags | Modifiers.FLAG_ABSTRACT;
+    // Only check for getters. The test can only fail if there is both a setter
+    // and a getter with the same name, and we only need to check each abstract
+    // field once, so we just ignore setters.
+    if (!member.isGetter()) return;
+
+    // Find the associated abstract field.
+    ClassElement classElement = member.getEnclosingClass();
+    Element lookupElement = classElement.lookupLocalMember(member.name);
+    if (lookupElement === null) {
+      compiler.internalErrorOnElement(member,
+                                      "No abstract field for accessor");
+    } else if (lookupElement.kind !== ElementKind.ABSTRACT_FIELD) {
+       compiler.internalErrorOnElement(
+           member, "Inaccessible abstract field for accessor");
+    }
+    AbstractFieldElement field = lookupElement;
+
+    if (field.getter === null) return;
+    if (field.setter === null) return;
+    int getterFlags = field.getter.modifiers.flags | Modifiers.FLAG_ABSTRACT;
+    int setterFlags = field.setter.modifiers.flags | Modifiers.FLAG_ABSTRACT;
     if (getterFlags !== setterFlags) {
       final mismatchedFlags =
         new Modifiers.withFlags(null, getterFlags ^ setterFlags);
       compiler.reportMessage(
-          compiler.spanFromElement(member.getter),
+          compiler.spanFromElement(field.getter),
           MessageKind.GETTER_MISMATCH.error([mismatchedFlags]),
           api.Diagnostic.ERROR);
       compiler.reportMessage(
-          compiler.spanFromElement(member.setter),
+          compiler.spanFromElement(field.setter),
           MessageKind.SETTER_MISMATCH.error([mismatchedFlags]),
           api.Diagnostic.ERROR);
     }
   }
 
   reportErrorWithContext(Element errorneousElement,
                          MessageKind errorMessage,
                          Element contextElement,
                          MessageKind contextMessage) {
     compiler.reportMessage(
@@ skipping 2220 matching lines @@
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) {
     return library.find(name);
   }
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
   String toString() => '$element';
 }