Be less aggressive in sharing contexts between scopes so that sibling contexts

Be less aggressive in sharing contexts between scopes so that sibling contexts
cannot appear in nested node sequences (issue 22858), since this case cannot
be handled by the graph builder.
Add assert to graph builder to detect such occurences (uncovered async* bug).
Fix an async* issue where a scope was mistakenly reused.
Improve ast printing for node sequences.
Add regression test.

R=hausner@google.com, srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44592

[regis, 2015-03-19 22:06:07]

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

[hausner, 2015-03-19 22:32:55]

LGTM.

[srdjan, 2015-03-19 22:34:44]

lgtm

https://codereview.chromium.org/1020893004/diff/1/tests/language/regress_2285...
File tests/language/regress_22858_test.dart (right):

https://codereview.chromium.org/1020893004/diff/1/tests/language/regress_2285...
tests/language/regress_22858_test.dart:24: f1();
Indent 2 spaces

[regis, 2015-03-19 22:38:53]

Thank you both!

https://codereview.chromium.org/1020893004/diff/1/tests/language/regress_2285...
File tests/language/regress_22858_test.dart (right):

https://codereview.chromium.org/1020893004/diff/1/tests/language/regress_2285...
tests/language/regress_22858_test.dart:24: f1();
On 2015/03/19 22:34:44, srdjan wrote:
> Indent 2 spaces

Done.

[regis, 2015-03-19 22:39:56]

Committed patchset #2 (id:20001) manually as r44592 (presubmit successful).