Fix a bug in the async translation of let expressions.

Fix a bug in the async translation of let expressions.

If the body of a let expression contains an await, the translation could
hoist statements that contain references to the let-bound variable out
of its scope.

Instead split by cases on whether the body contains an await.  If so, we
hoist let-bound variables to the statement level.  If not, we produce a
let expression.

BUG=
R=asgerf@google.com

Committed: https://github.com/dart-lang/kernel/commit/a33c09fb981e5830cae1efc482b06cf19f89291b

[Kevin Millikin (Google), 2016-11-04 13:10:11]

kmillikin@google.com changed reviewers:
+ asgerf@google.com

[Kevin Millikin (Google), 2016-11-04 13:10:12]

I will add a test case using the VM when I roll this into the SDK.

[asgerf, 2016-11-04 13:15:03]

LGTM

[Kevin Millikin (Google), 2016-11-04 13:20:17]

Description was changed from

==========
Fix a bug in the async translation of let expressions.

If the body of a let expression contains an await, the translation could
hoist statements that contain references to the let-bound variable out
of its scope.

Instead split by cases on whether the body contains an await.  If so, we
hoist let-bound variables to the statement level.  If not, we produce a
let expression.

BUG=
==========

to

==========
Fix a bug in the async translation of let expressions.

If the body of a let expression contains an await, the translation could
hoist statements that contain references to the let-bound variable out
of its scope.

Instead split by cases on whether the body contains an await.  If so, we
hoist let-bound variables to the statement level.  If not, we produce a
let expression.

BUG=
R=asgerf@google.com

Committed:
https://github.com/dart-lang/kernel/commit/a33c09fb981e5830cae1efc482b06cf19f...
==========

[Kevin Millikin (Google), 2016-11-04 13:20:18]

Committed patchset #2 (id:20001) manually as
a33c09fb981e5830cae1efc482b06cf19f89291b (presubmit successful).