Futures clobber exception stacks

[nex3]

Consider the following code:

main() {
  new Future.immediate(null).chain(() {
    throw 'error';
  }).then(() {});
}

The trace and line information of the original exception is not reported. Instead, it's reported as being thrown from within the call to "then". This makes it extremely difficult to debug errors in asynchronous code that uses futures heavily, since it's impossible to tell where an exception was originally thrown.

[nex3]

It's probably worth noting that this is on the VM. I haven't tested on Frog or dart2js.

[nex3]

I've figured out a way around this, although I'm not sure how reliable it is. If you replace "catch (final e)" with "catch (Exception e)" in futures_implementation.dart, the correct stack trace is used.

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cc @sigmundch.

[nex3]

Delving into this a little deeper, it seems my workaround is not functional. All it was doing was causing the exception to avoid the catch handler and hit the top level. While this did provide the correct stack trace, it also avoided all the Future exception-handling infrastructure.

The fundamental problem here is that it's not possible to re-raise an exception with its original stack trace, so when Future encounters the call to "then" it can't do anything about its old stack.

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Added Duplicate label.
Marked as being merged into #3974.