Add notion of current target isolate to ddbg.dart

Add notion of current target isolate to ddbg.dart

Command li lists isolates in a vm and marks the current
target isolate with an asterisk. Command sci sets the current
target isolate to a particular isolate id.

li
List of isolates:
  7115 paused at file: typed_message_test.dart lib: 12 token: 59
* 7112 running
sci 7115
Setting current target isolate to 7115
li
List of isolates:
* 7115 paused at file: typed_message_test.dart lib: 12 token: 59
  7112 running

R=turnidge@google.com

Committed: https://code.google.com/p/dart/source/detail?r=30595

[turnidge, 2013-11-22 19:24:00]

LGTM with comments.

https://codereview.chromium.org/83743003/diff/1/tools/ddbg.dart
File tools/ddbg.dart (right):

https://codereview.chromium.org/83743003/diff/1/tools/ddbg.dart#newcode130
tools/ddbg.dart:130: currentIsolate.pausedLocation = null;
I know that the sendCmd cannot complete immediately, but if it could, then this
statement could overwrite pausedLocation.  Maybe move this line somewhere above
the call to sendCmd?

https://codereview.chromium.org/83743003/diff/1/tools/ddbg.dart#newcode552
tools/ddbg.dart:552: mainIsolate = currentIsolate = null;
So terminating the main isolate always sets current isolate to null?  Is this
right?

[hausner, 2013-11-22 22:54:42]

https://codereview.chromium.org/83743003/diff/1/tools/ddbg.dart
File tools/ddbg.dart (right):

https://codereview.chromium.org/83743003/diff/1/tools/ddbg.dart#newcode130
tools/ddbg.dart:130: currentIsolate.pausedLocation = null;
On 2013/11/22 19:24:00, turnidge wrote:
> I know that the sendCmd cannot complete immediately, but if it could, then
this
> statement could overwrite pausedLocation.  Maybe move this line somewhere
above
> the call to sendCmd?
Good point. Created a new handler that sets the paused location to null when the
command is confirmed.

https://codereview.chromium.org/83743003/diff/1/tools/ddbg.dart#newcode552
tools/ddbg.dart:552: mainIsolate = currentIsolate = null;
On 2013/11/22 19:24:00, turnidge wrote:
> So terminating the main isolate always sets current isolate to null?  Is this
> right?
Probably not. The main isolate can terminate and the VM stays alive. Should
probably just pick a random isolate and set it to be the current target isolate.
Done.

[hausner, 2013-11-22 22:55:09]

Committed patchset #3 manually as r30595 (presubmit successful).