Fix crashing bugs when dart code executed inside the debugger triggers breakpoints

Cleanup scope auto entry-exit code to reduce boilerplate slightly. Disable all cases where the debugger could trigger a breakpoint while already in the debugger to avoid Dartium crashers on triggering the debugger while already in the debugger.

BUG=
R=vsm@google.com

Committed: https://src.chromium.org/viewvc/multivm?view=rev&revision=1324

[Jacob, 2013-08-02 16:52:59]

Modify DartHandleProxy so that invoking Dart code from within a proxy never
triggers the debugger.  In general this shouldn't be required but for now it is
due to crashing behavior when DartVM triggers a breakpoint while stopped at a
breakpoint.
Modify interop code to avoid throwing a NSM when an invalid property name is
accessed.  This insures that JS feature detection code still works when
operating on Dart objects and better matches future plans for general JS-Dart
Interop.

Cleanup scope auto entry-exit code to reduce boilerplate slightly.

[vsm, 2013-08-02 19:56:47]

lgtm

[Jacob, 2013-08-02 20:07:57]

Committed patchset #1 manually as r1324 (presubmit successful).