Fix uncaught embedded exceptions on gist loading

[jcollins-g]

Partial fix for #791, at least restoring things to where they were before this cropped up.
@kwalrath

This fixes errors of this form:

async_patch.dart:726 Uncaught Error: DetailedApiRequestError(status: 400, message: main.dart:
Error: No 'main' method found.

Error: Compilation failed.
)
    at Object.a (js_helper.dart:1759)
    at yj.dart.yj.$1 (clients.dart:881)
    at ww.fH (zone.dart:1381)
    at fP.md (future_impl.dart:129)
    at vA.$0 (future_impl.dart:633)
    at Object.ca (future_impl.dart:662)
    at F.aE (future_impl.dart:467)
    at Object.dh (stream_pipe.dart:63)
    at tE.dart.tE.$1 (stream.dart:1149)
    at ww.fI (zone.dart:1316)

They were caused by compilation/running not waiting on the asynchronous gist loading before executing. While the code was always suspect, we might have accidentally not hit this because of implementation details in Dart, and that changed with 2.x.