Support async expression evaluation in debuggers

[DanTup]

Originally raised by @Rodsevich at https://github.com/Dart-Code/Dart-Code/issues/2212. Evalating any async code just returns the Future. The code doesn't actually run because while the isolate is paused, the event/microtask queue is not being processed.

Wrapping waitFor() around the expression doesn't work either, as it returns "waitForEventSync is not supported by this embedder".

Based on @bkonyi's comments this might be non-trivial to support, but it seems like a nice feature (given how common async code is). I think it's worth having an issue for people to collect 👍's.

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When running Future returning code in the debug console, only a

_Future (Instance of 'Future')

instance is returned. And no way of interacting with it.

It doesn't even works the .then((s){print('funcó: $s');}) call

[DanTup]

It seems like the .then() will work, but only once you resume (since the isolate is paused so the async code doesn't actually run until it's resumed).