Fix debugger test that broke with the change to the json library.

tests/standalone/debugger/debug_lib.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.
 
 // Library used by debugger wire protocol tests (standalone VM debugging).
 
 library DartDebugger;
 
 import "dart:io";
 import "dart:utf";
-import "dart:json";
+import "dart:json" as JSON;
 
 // TODO(hausner): need to select a different port number for each
 // test that runs in parallel.
 var debugPort = 5860;
 
 // Whether or not to print debug target process on the console.
 var showDebuggeeOutput = true;
 
 // Whether or not to print the debugger wire messages on the console.
 var verboseWire = false;
@@ skipping 490 matching lines @@
   if (showDebuggeeOutput) targetOpts.add("--verbose_debug");
   targetOpts.add(options.script);
   targetOpts.add("--debuggee");
 
   Process.start(options.executable, targetOpts).then((Process process) {
     print("Debug target process started");
     process.stdin.close();
     process.stdout.onData = process.stdout.read;
     process.stderr.onData = process.stderr.read;
     process.onExit = (int exitCode) {
+      Expect.equals(0, exitCode);
       print("Debug target process exited with exit code $exitCode");
     };
     var debugger = new Debugger(process, debugPort);
     stdin.onClosed = () => debugger.close();
     stdin.onError = (error) => debugger.close();
     debugger.runScript(script);
   });
   return true;
 }