Support overriding fields with fields.

dart/lib/compiler/implementation/world.dart

Index: dart/lib/compiler/implementation/world.dart
diff --git a/dart/lib/compiler/implementation/world.dart b/dart/lib/compiler/implementation/world.dart
index 88480cdf6765be1e63d32c0587643327d6fd3bc7..57319da3d35459f43dda0aed02290b94de2d0f04 100644
--- a/dart/lib/compiler/implementation/world.dart
+++ b/dart/lib/compiler/implementation/world.dart
@@ -52,7 +52,7 @@ class World {
 
   bool isOnlyFields(Type type, SourceString member) {
     MemberSet memberSet = _memberSetFor(type, member);
-    return !memberSet.isEmpty() && memberSet.hasJustFields();
+    return memberSet.hasExactlyOneField();
   }
 }
 
@@ -71,7 +71,16 @@ class MemberSet {
 
   bool isEmpty() => elements.isEmpty();
 
-  bool hasJustFields() {
-    return elements.every((Element element) => element.isField());
+  bool hasExactlyOneField() {
+    int fieldCount = 0;
+    int nonFieldCount = 0;
+    elements.forEach((Element element) {
+      if (element.isField()) {
+        fieldCount++;
+      } else {
+        nonFieldCount++;
+      }
+    });
+    return fieldCount == 1 && nonFieldCount == 0;
   }
 }