Fix isolate lifecycle handling in service library

Fix isolate lifecycle handling in service library

- Use IsolateCreated and IsolateShutdown events to edit the VM's isolate cache directly.
- IsolateCreated and IsolateShutdown events are owned by the VM.
- When eagerly loading a new isolate, consume any errors rather than letting them bubble up to the application level.
- When reloading the VM perform an edit the isolate cache instead of replacing it.
- When reloading the VM do not reload all isolates.
- Fix flaky coverage_test fail by delaying setting the breakpoint.
- Add isolate lifecycle unit test.
- Make the isolate cache observable and send an observable notification that the 'isolates' property has changed when editing the isolate cache.

R=rmacnak@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44056

[Cutch, 2015-02-25 21:09:36]

johnmccutchan@google.com changed reviewers:
+ rmacnak@google.com

[rmacnak, 2015-02-25 22:44:12]

LGTM

[Cutch, 2015-02-26 17:01:07]

Committed patchset #2 (id:20001) manually as 44056 (presubmit successful).