Change sdk/bin/dart2js to find an available build.
Change sdk/bin/dart2js to find an available build.

This parallels the change to sdk/bin/dart made by commit
ac8a228d0b1dc86cb02f9d15ce0320c4e03b56cd.

R=floitsch@google.com

Review URL: https://codereview.chromium.org/1650383002 .

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