Ensure unoptimized code when collecting awaiter return stack trace.
Ensure unoptimized code when collecting awaiter return stack trace.

Fixes crashes when running reload stress tests with the async stack trace stress flag
(python tools/test.py --hot-reload -t240 --builder-tag no_ipv6 language/await_exceptions_test)

R=johnmccutchan@google.com

Also: Use iterators to get function of a stack frame (refactoring only).
Review-Url: https://codereview.chromium.org/2761143004 .

dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk