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

lib/compiler/implementation/world.dart

Index: lib/compiler/implementation/world.dart
diff --git a/lib/compiler/implementation/world.dart b/lib/compiler/implementation/world.dart
index afef86402415900c9b4bb30aeb5842ffc6543ec1..f8c0519d9f34cf8c1d94c83924412146ebc2fc1e 100644
--- a/lib/compiler/implementation/world.dart
+++ b/lib/compiler/implementation/world.dart
@@ -31,7 +31,7 @@ class World {
   /**
    * Returns a [MemberSet] that contains the possible targets of a
    * selector named [member] accessible from the given [library] on a receiver
-   * with the given [type].
+   * with the given [type]. This includes all sub types.
    *
    * The [library] can be `null` if the [member] is not private.
    */
@@ -58,26 +58,23 @@ class World {
   }
 
   /**
-   * Returns the single field with the given [name] accessible in the given
-   * [library]. If there is no such field, or there are multiple possible
-   * fields returns `null`.
+   * Returns the field in [type] with the given [name] accessible in the given
+   * [library]. If no such field exists, or a subclass overrides the field
+   * returns `null`.
    */
   VariableElement locateSingleField(Type type,
                                     LibraryElement library,
                                     SourceString name) {
+    ClassElement cls = type.element;
+    Element result = cls.lookupMemberInLibrary(name, library);
+    if (result == null) return null;
+    if (!result.isField()) return null;
+
+    // Verify that no subclass overrides the field.
     MemberSet memberSet = _memberSetFor(type, library, name);
-    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;
+    if (memberSet.elements.length != 1) return null;
+    assert(memberSet.elements.contains(result));
+    return result;
   }
 
   Set<ClassElement> findNoSuchMethodHolders(Type type) {