VM: Fix bug with ??= expressions using await.

VM: Fix bug with ??= expressions using await.

Moving the construction of ?? into the parser using LetNode, so that the await-transformations are properly applied.

a ?? b becomes { temp = a; temp !== null ? temp : b; }

a ??= b becomes { temp = a; temp != null ? temp : a = b; }

BUG=issue #24392
R=hausner@google.com

Committed: https://github.com/dart-lang/sdk/commit/598018bd212a072e025866865ba4e5a1bd6841c7

[Florian Schneider, 2015-11-03 16:26:52]

Description was changed from

==========
VM: Fix bug with ??= expressions using await.

BUG=issue #24392
==========

to

==========
VM: Fix bug with ??= expressions using await.

Moving the construction of ?? into the parser using LetNode, so that the
await-transformations are properly applied.

a ?? b becomes { temp = a; temp !== null ? temp : b; }

a ??= b becomes { temp = a; temp != null ? temp : a = b; }

BUG=issue #24392
==========

[Florian Schneider, 2015-11-03 16:49:59]

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

[sra1, 2015-11-03 17:02:55]

sra@google.com changed reviewers:
+ sra@google.com

[sra1, 2015-11-03 17:02:55]

DBC - please add a test so we never do this by accident in dart2js.

[hausner, 2015-11-03 19:03:08]

Thank you for fixing this.

As discussed offline, I can't remember why I handled the ?? nodes in the
backend. I tried your approach with conditional expressions first. It seems to
work fine, though.

Another way of fixing this bug would be to leave everything as is, but handle
the short-circuit nature of the ?? operator properly in ast_transformer.cc. It
already does the right thing for boolean && and ||.

LGTM but please do add a regression case to the tests.

https://codereview.chromium.org/1417733007/diff/40001/runtime/vm/parser.cc
File runtime/vm/parser.cc (right):

https://codereview.chromium.org/1417733007/diff/40001/runtime/vm/parser.cc#ne...
runtime/vm/parser.cc:10760: LetNode* let_node = new(Z) LetNode(no_pos);
Should be possible to call OptimizeBinaryOpNode() here and re-use that code.

[Florian Schneider, 2015-11-03 19:10:59]

Added test.

https://codereview.chromium.org/1417733007/diff/40001/runtime/vm/parser.cc
File runtime/vm/parser.cc (right):

https://codereview.chromium.org/1417733007/diff/40001/runtime/vm/parser.cc#ne...
runtime/vm/parser.cc:10760: LetNode* let_node = new(Z) LetNode(no_pos);
On 2015/11/03 19:03:08, hausner wrote:
> Should be possible to call OptimizeBinaryOpNode() here and re-use that code.

Yes, good point.

[Florian Schneider, 2015-11-03 20:47:10]

Committed patchset #4 (id:60001) manually as
598018bd212a072e025866865ba4e5a1bd6841c7 (presubmit successful).