Use correct scope to lookup saved async context variable.

Use correct scope to lookup saved async context variable.
Stop inserting aliases in all intermediary scopes for explicitly captured
variables.
Use proper lookup function in async code to access async completer variable.
Add asserts verifying async variables existence.

R=hausner@google.com

Committed: https://github.com/dart-lang/sdk/commit/a799659bcfd1d36593539390404b20ca09122223

[regis, 2015-08-25 22:09:11]

regis@google.com changed reviewers:
+ hausner@google.com

[regis, 2015-08-25 22:09:12]

This is a first round of cleanups before trying to reduce the number of captured
variables in async code.

Thanks

[hausner, 2015-08-25 22:44:47]

lgtm

[regis, 2015-08-25 22:46:22]

Committed patchset #1 (id:1) manually as
a799659bcfd1d36593539390404b20ca09122223 (presubmit successful).