Fix tests broken by proxy changes.

runtime/vm/snapshot_test.dart

 // Copyright (c) 2011, 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 Fields  {
   Fields(int i, int j) : fld1 = i, fld2 = j, fld5 = true {}
   int fld1;
   final int fld2;
   static int fld3;
   static final int fld4 = 10;
@@ skipping 1362 matching lines @@
 // THE REST OF THIS FILE COULD BE AUTOGENERATED
 // ---------------------------------------------------------------------------
 
 interface Mint$Proxy {
 
   Purse$Proxy createPurse(int balance);  // Promise<int> balance.
 
 }
 
 
-class Mint$ProxyImpl extends Proxy implements Mint$Proxy {
+class Mint$ProxyImpl extends ProxyImpl implements Mint$Proxy {
 
   Mint$ProxyImpl(Proxy isolate) : super.forReply(isolate.call([null])) {}
 
   Purse$Proxy createPurse(int balance) {
     return new Purse$ProxyImpl(this.call([balance]));
   }
 
 }
 
 
@@ skipping 29 matching lines @@
 
 interface Purse$Proxy {
 
   Promise<int> queryBalance();
   Purse$Proxy sproutPurse();
   void deposit(int amount, Purse$Proxy source);  // Promise<int> amount.
 
 }
 
 
-class Purse$ProxyImpl extends Proxy implements Purse$Proxy {
+class Purse$ProxyImpl extends ProxyImpl implements Purse$Proxy {
 
   Purse$ProxyImpl(Promise<SendPort> port) : super.forReply(port) { }
 
   Promise<int> queryBalance() {
     return this.call(["balance"]);
   }
 
   void deposit(int amount, Purse$Proxy source) {
     this.send(["deposit", amount, source]);
   }
@@ skipping 745 matching lines @@
       });
     });
   }
 
   void main() {
     this.port.receive((ignored, replyTo) {
       replyTo.send("foo", null);
     });
   }
 }