Fixes a problem with the recovery of contexts in the debugger at closure calls.

Fixes a problem with the recovery of contexts in the debugger at closure calls.

When we call a closure, the caller saves the context before the call.
So when walking the stack, we used to check the callee activation to
see if it was a closure call, and then recover our caller-saved
context.

This breaks down in the following case:

1. Caller calls function F.
2. Function F is compiled.
3. The compiler needs to run code to evaluate a constant (e.g. a
   static final getter).
4. We hit a breakpoint *while* evaluating the constant.

In this case, there is no callee frame for the closure call yet.

I fix this by using the the pc descriptors to see if the current pc in
the caller is a closure call.  This avoids needing to have a callee
frame available.

---

ALSO:

New flags:

--verify-incoming-contexts: in development - verify assumptions about
  contexts in stub code. (currently ia32 only)

--trace-debugger-stacktrace: this helps debug these kinds of
  stacktrace issues.

Miscellaneous improvements to frame and context printing.

BUG=
R=johnmccutchan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35130

[Cutch, 2014-04-16 19:50:16]

lgtm

[turnidge, 2014-04-16 20:03:36]

Committed patchset #2 manually as r35130 (presubmit successful).