Support overriding fields with fields.

dart/lib/compiler/implementation/world.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.
 
 class World {
   final Map<ClassElement, Set<ClassElement>> subtypes;
 
   World() : subtypes = new Map<ClassElement, Set<ClassElement>>();
 
   void populate(Compiler compiler, Collection<LibraryElement> libraries) {
@@ skipping 32 matching lines @@
     Set<ClassElement> subtypesOfCls = subtypes[cls];
     if (subtypesOfCls !== null) {
       for (ClassElement sub in subtypesOfCls) {
         element = sub.lookupLocalMember(member);
         if (element !== null) result.add(element);
       }
     }
     return result;
   }
 
-  bool isOnlyFields(Type type, SourceString member) {
+  /**
+   * Returns the single field with the given name, if such a field
+   * exists. If there are multple fields, or none, return null.
+   */
+  VariableElement locateSingleField(Type type, SourceString member) {
     MemberSet memberSet = _memberSetFor(type, member);
-    return !memberSet.isEmpty() && memberSet.hasJustFields();
+    int fieldCount = 0;
+    int nonFieldCount = 0;
+    VariableElement field;
+    memberSet.elements.forEach((Element element) {
+      if (element.isField()) {
+        field = element;
+        fieldCount++;
+      } else {
+        nonFieldCount++;
+      }
+    });
+    return (fieldCount == 1 && nonFieldCount == 0) ? field : null;
   }
 }
 
 /**
  * A [MemberSet] contains all the possible targets for a selector.
  */
 class MemberSet {
   final Set<Element> elements;
   final SourceString name;
 
   MemberSet(SourceString this.name) : elements = new Set<Element>();
 
   void add(Element element) {
     elements.add(element);
   }
 
   bool isEmpty() => elements.isEmpty();
-
-  bool hasJustFields() {
-    return elements.every((Element element) => element.isField());
-  }
 }