Fix the tests.

It seems we tickled a core library inconsistency (dart-lang/sdkdart-lang/json_rpc_2#25655):
although on the VM `FormatException.source` is set for JSON parse
errors, it is not set using dart2js.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1659313002 .