Fix for issues 6080 - pass in the Isolate's top context into the stub for invoking dart code from n…

Fix for issues 6080 -
- Read the saved context from the entry frame when straddling across C++ frames while iterating over a stack trace in the debugger. This ensures that the correct context is setup in the ActivationFrame structure in these scenarios (instead of the empty context).

- Read the saved context from the caller's frame when iterating over a stack trace in the debugger. The compiler saves the context in the caller frame before invoking closures, we read this saved context when iterating the stack trace.
Committed: https://code.google.com/p/dart/source/detail?r=18235

[hausner, 2013-02-07 17:00:45]

LGTM with one comment for "optimization".

https://chromiumcodereview.appspot.com/12179020/diff/15002/runtime/vm/parser.cc
File runtime/vm/parser.cc (right):

https://chromiumcodereview.appspot.com/12179020/diff/15002/runtime/vm/parser....
runtime/vm/parser.cc:183: // Allocate a local variable to save the current
context when we call into
Can we allocate this variable only when really needed? That would save a stack
slot in almost all activation frames, and one VarDescriptor entry in pretty much
every function object.

[siva, 2013-02-07 22:08:31]

https://chromiumcodereview.appspot.com/12179020/diff/15002/runtime/vm/parser.cc
File runtime/vm/parser.cc (right):

https://chromiumcodereview.appspot.com/12179020/diff/15002/runtime/vm/parser....
runtime/vm/parser.cc:183: // Allocate a local variable to save the current
context when we call into
Changed this to allocate only when needed following the expression temp model.

PTAL.

On 2013/02/07 17:00:45, hausner wrote:
> Can we allocate this variable only when really needed? That would save a stack
> slot in almost all activation frames, and one VarDescriptor entry in pretty
much
> every function object.

[hausner, 2013-02-07 22:19:15]

LGTM. This even fixed a little buglet where the expression temp variable was
unnecessarily allocated :)