Running tests on d8 has gotten more annoying

[Peter Ahé]

I've made some changes to the JavaScript implementation of dart:isolate. As you may know, you only have to use a timer-based future to get the dart:isolate library imported implicitly by dart2js.

These changes mean that there is a greater chance that you'll need to load sdk/lib/_internal/lib/preambles/d8.js for dart2js output to work.

Cheers,

Peter