Fix context level info in for-loops

Fix context level info in for-loops

The stack overflow check on the back-branch of the loop must
be at the same context level as the (outer) loop block of
the for-loop.

Before this change, the stack check was recorded to be at a
token position that is outside of the loop block, which caused
a context mismatch on some stack traces when the debugger
or the observatory interrupted the isolate on that back
branch.

It is not practical to write a test for this, unfortunately.

BUG=25684
R=fschneider@google.com

Committed: https://github.com/dart-lang/sdk/commit/3107a41bb7540156eb0ac4dabfa16d878cd72134

[hausner, 2016-03-04 23:38:16]

Description was changed from

==========
Fix context level info in for-loops

The stack overflow check on the back-branch of the loop must
be at the same context level as the (outer) loop block of
the for-loop.

Before this change, the stack check was recorded to be at a
token position that is outside of the loop block, which caused
a context mismatch on some stack traces when the debugger
or the observatory interrupted the isolate on that back
branch.

It is not practical to write a test for this, unfortunately.

BUG=25684
==========

to

==========
Fix context level info in for-loops

The stack overflow check on the back-branch of the loop must
be at the same context level as the (outer) loop block of
the for-loop.

Before this change, the stack check was recorded to be at a
token position that is outside of the loop block, which caused
a context mismatch on some stack traces when the debugger
or the observatory interrupted the isolate on that back
branch.

It is not practical to write a test for this, unfortunately.

BUG=25684
==========

[hausner, 2016-03-04 23:38:16]

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

[Florian Schneider, 2016-03-05 00:10:42]

Lgtm.

It may be possible to reliably reproduce this crash with --stacktrace-every=N
stack checks, using a small N.

[hausner, 2016-03-05 00:37:07]

Description was changed from

==========
Fix context level info in for-loops

The stack overflow check on the back-branch of the loop must
be at the same context level as the (outer) loop block of
the for-loop.

Before this change, the stack check was recorded to be at a
token position that is outside of the loop block, which caused
a context mismatch on some stack traces when the debugger
or the observatory interrupted the isolate on that back
branch.

It is not practical to write a test for this, unfortunately.

BUG=25684
==========

to

==========
Fix context level info in for-loops

The stack overflow check on the back-branch of the loop must
be at the same context level as the (outer) loop block of
the for-loop.

Before this change, the stack check was recorded to be at a
token position that is outside of the loop block, which caused
a context mismatch on some stack traces when the debugger
or the observatory interrupted the isolate on that back
branch.

It is not practical to write a test for this, unfortunately.

BUG=25684
R=fschneider@google.com

Committed:
https://github.com/dart-lang/sdk/commit/3107a41bb7540156eb0ac4dabfa16d878cd72134
==========

[hausner, 2016-03-05 00:37:08]

Committed patchset #1 (id:1) manually as
3107a41bb7540156eb0ac4dabfa16d878cd72134 (presubmit successful).

[hausner, 2016-03-05 00:40:24]

Thank you.

Even with the --stacktrace-every=N option, the test would be unreliable. If it
doesn't fail, it hangs. If you make it terminate after a set time, you still
don't know whether the bug is indeed fixed.

Unreliable tests are worse than no tests :-)