Keep track of current "pause" event in the debugger.

Keep track of current "pause" event in the debugger.

This will allow us (later) to make nice descriptions of the state of
an isolate, e.g. "isolate 7112 is paused due to breakpoint 2" or
"isolate 7112 is paused due to interrupt" or "isolate 7112 is
running".

Use a new Pause() function to call the event handler during a pause.
It does some shared checking, setup, teardown.

Minor change in isolate interrupt handling - before we were not
setting 'in_event_handler_' in this case.  Now we set 'pause_event_'
here, which will guard against recursive pauses in this case.

Reviewer: not sure what I think about all the names I used here.
"pause_event_" is very similar to PausedEvent but means something
different, as it includes pauses, interrupts, and (maybe) exceptions.
Dunno.

R=hausner@google.com

Committed: https://code.google.com/p/dart/source/detail?r=31356

[turnidge, 2013-12-18 21:27:58]

https://codereview.chromium.org/118603002/diff/1/runtime/vm/debugger.cc
File runtime/vm/debugger.cc (right):

https://codereview.chromium.org/118603002/diff/1/runtime/vm/debugger.cc#newco...
runtime/vm/debugger.cc:191: ASSERT(debugger->stack_trace_ == NULL);
in_event_handler_ was missing here before.  I think this was a bug.

[turnidge, 2013-12-18 21:38:19]

Oops, forgot to add the reviewer...

Matthias, you're it.

[hausner, 2013-12-18 22:56:10]

lgtm

[turnidge, 2013-12-20 22:30:28]

Committed patchset #3 manually as r31356 (presubmit successful).