Some small changes to isolate handling in ddbg.

Some small changes to isolate handling in ddbg.

- Whenever we hit a breakpoint or other interruption, change the
  current isolate.

- The first isolate to pause is now the main isolate.  (Instead of
  first created.)

R=hausner@google.com

Committed: https://code.google.com/p/dart/source/detail?r=31142

[hausner, 2013-12-13 18:41:40]

lgtm

[turnidge, 2013-12-13 18:58:39]

On 2013/12/13 18:41:40, hausner wrote:
> lgtm

Added error checking for cases where there is no current isolate based on our
conversation.  Please take another look.

Sorry that the error check is a bit repetitive.  As more commands migrate to the
new command dispatch, this should look better (or at least different...)

[hausner, 2013-12-13 19:03:05]

Still looks good to me.

[turnidge, 2013-12-13 19:13:52]

Committed patchset #5 manually as r31142 (presubmit successful).