Fix context handling in for-in loops

Fix context handling in for-in loops

The creation and assignment of a fresh loop variable appeared to be outside
of the loop body. When the assignment fails, e.g. due to a type check, the
stack walking code could thus access the wrong context variables.

This CL fixes code and PC descriptor generation for for-in loops and also
fixes a separate bug that crashed the VM when printing an internal error
message.

Fixes issue 20999.

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40552

[hausner, 2014-09-22 16:13:19]

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

[regis, 2014-09-22 16:43:42]

LGTM

A regression test would be nice, although I guess it is not easy to setup.

[hausner, 2014-09-22 17:22:59]

A test would have to assume a lot of VM-internal implementation details and
would not be able to check whether the assumptions are correct. When the
assumptions no longer hold, the test might become ineffective or even pass for
the wrong reasons.

[hausner, 2014-09-22 17:23:52]

Committed patchset #1 (id:1) manually as r40552 (presubmit successful).