Fix stepping in when paused in async functions.

Fix stepping in when paused in async functions.

R=rmacnak@google.com

Committed: https://github.com/dart-lang/sdk/commit/ab6d7bba5372205d9a5a7f4e2177fface5696def

[sivachandra, 2017-08-14 15:23:30]

sivachandra@google.com changed reviewers:
+ asiva@google.com, rmacnak@google.com

[sivachandra, 2017-08-14 15:27:06]

I spent some time to fix this issue:
https://github.com/dart-lang/sdk/issues/30240

I am not able to reproduce the exact issue as reported there, but I am able to
repro a VM crash when we pause and "step in" with the example the OP has posted.
This change fixes the crash, but I am not sure if this is the correct approach.
I and sending it out so that you can explain why it is correct or not correct.

[rmacnak, 2017-08-14 20:15:24]

LGTM

Note this fixes any kind of step when paused at the root of the message loop,
where there is not yet any Dart frame.

https://codereview.chromium.org/2994983002/diff/1/runtime/vm/debugger.cc
File runtime/vm/debugger.cc (right):

https://codereview.chromium.org/2994983002/diff/1/runtime/vm/debugger.cc#newc...
runtime/vm/debugger.cc:3272: if (stack_trace_->Length() > 0 &&
if ((stack_trace_->Length() > 0) &&

[sivachandra, 2017-08-15 12:14:16]

Description was changed from

==========
Fix stepping in when paused in async functions.
==========

to

==========
Fix stepping in when paused in async functions.

R=rmacnak@google.com

Committed:
https://github.com/dart-lang/sdk/commit/ab6d7bba5372205d9a5a7f4e2177fface5696def
==========

[sivachandra, 2017-08-15 12:14:17]

Committed patchset #2 (id:20001) manually as
ab6d7bba5372205d9a5a7f4e2177fface5696def (presubmit successful).