Fix access to fields that didn't exist in the class.

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 {
   Compiler compiler;  // Set in populate().
   final Map<ClassElement, Set<ClassElement>> subtypes;
 
   World() : subtypes = new Map<ClassElement, Set<ClassElement>>();
 
@@ skipping 12 matching lines @@
 
     compiler.resolverWorld.instantiatedClasses.forEach(addSubtypes);
 
     // Mark the world as populated.
     assert(compiler !== null);
     this.compiler = compiler;
   }
 
   /**
    * Returns a [MemberSet] that contains the possible targets of the given
-   * [selector] on a receiver with the given [type].
+   * [selector] on a receiver with the given [type]. This includes all sub
+   * types.
    */
   MemberSet _memberSetFor(Type type, Selector selector) {
     assert(compiler !== null);
     ClassElement cls = type.element;
     SourceString name = selector.name;
     LibraryElement library = selector.library;
     MemberSet result = new MemberSet(name);
     Element element = cls.lookupSelector(selector);
     if (element !== null) result.add(element);
 
     bool isPrivate = name.isPrivate();
     Set<ClassElement> subtypesOfCls = subtypes[cls];
     if (subtypesOfCls !== null) {
       for (ClassElement sub in subtypesOfCls) {
         // Private members from a different library are not visible.
         if (isPrivate && sub.getLibrary() != library) continue;
         element = sub.lookupLocalMember(name);
         if (element !== null) result.add(element);
       }
     }
     return result;
   }
 
   /**
-   * Returns the single field with the given [selector]. If there is no such
-   * field, or there are multiple possible fields returns [:null:].
+   * Returns the field in [type] described by the given [selector].
+   * If no such field exists, or a subclass overrides the field
+   * returns [:null:].
    */
   VariableElement locateSingleField(Type type, Selector selector) {
     MemberSet memberSet = _memberSetFor(type, selector);
-    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;
+    ClassElement cls = type.element;
+    Element result = cls.lookupSelector(selector);
+    if (result == null) return null;
+    if (!result.isField()) return null;
+
+    // Verify that no subclass overrides the field.
+    if (memberSet.elements.length != 1) return null;
+    assert(memberSet.elements.contains(result));
+    return result;
   }
 
   Set<ClassElement> findNoSuchMethodHolders(Type type) {
     Set<ClassElement> result = new Set<ClassElement>();
     Selector noSuchMethodSelector = new Selector.noSuchMethod();
     MemberSet memberSet = _memberSetFor(type, noSuchMethodSelector);
     for (Element element in memberSet.elements) {
       ClassElement holder = element.getEnclosingClass();
       if (holder !== compiler.objectClass &&
           noSuchMethodSelector.applies(element, compiler)) {
@@ skipping 12 matching lines @@
   final SourceString name;
 
   MemberSet(SourceString this.name) : elements = new Set<Element>();
 
   void add(Element element) {
     elements.add(element);
   }
 
   bool isEmpty() => elements.isEmpty();
 }