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

Added checking of the debugger subprocess exit code that would
have caught this.

R=whesse@google.com,hausner@google.com
BUG=

Review URL: https://codereview.chromium.org//12100003

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@17846 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk