Add the streamListen and streamCancel rpcs to the vm service.

Add the streamListen and streamCancel rpcs to the vm service.
    
    We currently support 5 streams: Isolate, Debug, GC, _Echo, and _Graph.
    
    Only generate events when at least one client is listening to the
    corresponding event stream.
    
    Only send events to the clients that actually request them.
    
    Implement this all as Dart streams in the service lib.
    
    Update tests.  Fix a race in async_generator_..._test that was biting me.

Committed: https://github.com/dart-lang/sdk/commit/0967b6307399b9296faa58b9b56eca045c9112d9

[turnidge, 2015-05-21 21:11:40]

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

[Cutch, 2015-05-26 20:37:42]

LGTMwC

https://codereview.chromium.org/1143783003/diff/40001/runtime/vm/service.cc
File runtime/vm/service.cc (right):

https://codereview.chromium.org/1143783003/diff/40001/runtime/vm/service.cc#n...
runtime/vm/service.cc:110: if (strcmp(stream_id, "Isolate") == 0) {
would be nice to have these up with the flags:

const char* isolate_events_stream_id = "Isolate"

[turnidge, 2015-05-26 20:52:40]

Committed patchset #4 (id:60001) manually as
0967b6307399b9296faa58b9b56eca045c9112d9 (presubmit successful).