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

tests/standalone/debugger/debug_lib.dart

Index: tests/standalone/debugger/debug_lib.dart
diff --git a/tests/standalone/debugger/debug_lib.dart b/tests/standalone/debugger/debug_lib.dart
index 82a115bed1e643428412c7d84db4ccbd6276937e..e3a332fc7a782d19edd4df58ee8e68825985bc0b 100644
--- a/tests/standalone/debugger/debug_lib.dart
+++ b/tests/standalone/debugger/debug_lib.dart
@@ -8,7 +8,7 @@ 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.
@@ -519,6 +519,7 @@ bool RunScript(List script) {
     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);