Make the json_rpc_2 server test dart2js-compatible.

Make the json_rpc_2 server test dart2js-compatible.

R=rnystrom@google.com
TBR

Committed: https://code.google.com/p/dart/source/detail?r=34226

[nweiz, 2014-03-21 02:34:10]

Committed patchset #1 manually as r34226 (presubmit successful).

[Bob Nystrom, 2014-03-21 17:27:42]

How does this make dart2js happier?

[nweiz, 2014-03-21 19:40:17]

On 2014/03/21 17:27:42, Bob Nystrom wrote:
> How does this make dart2js happier?

When we run dart2js-compiled code, the stack traces don't contain
"server_test.dart" because it's some compiled JavaScript filename instead.
dart2js's JSON parser also emits different error messages than the VM's, and I
had to parse out the JSON response to use a sufficiently flexible matcher.

[Bob Nystrom, 2014-03-21 19:43:16]

Ah, thanks. LGTM.