Move notification of isolate creation until after the ObjectStore is initialized.

The debugger_ is initialized in Dart::CreateIsolate but the object
store is only valid after Dart::InitializeIsolate.  I could have moved
the initalization of debugger_ later, but then many of the debugger()
accesses in the vm would need null checks.  Instead I moved the
isolate creation notification a bit.

I was only able to see this problem when running a two isolate program
with the --pause-isolates-on-start option.

BUG=
R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org//802543002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42344 260f80e4-7a28-3924-810f-c04153c831b5