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 .
3 files changed
tree: 3e1c9defdd17dc3881bf6c99831106e45640559e
  1. pkgs/