Eliminate dart2js warning about overriding `==`, but not `hashCode`.

Closes dart-lang/source_spandart-lang/source_span#5

R=nweiz@google.com

Review URL: https://codereview.chromium.org//1315423002 .
3 files changed
tree: ebe7c7e7ea3485f78546635db287abb92937517f
  1. pkgs/