Refactor, rename, and generally rationalize code in the Frog isolates library,

lib/isolate/frog/compiler_hooks.dart

Index: lib/isolate/frog/compiler_hooks.dart
===================================================================
--- lib/isolate/frog/compiler_hooks.dart	(revision 5269)
+++ lib/isolate/frog/compiler_hooks.dart	(working copy)
@@ -12,7 +12,7 @@
  * applications (e.g. hello world), this call is not emitted.
  */
 void startRootIsolate(entry) {
-  _globalState = new _GlobalState();
+  _globalState = new _Manager();
 
   // Don't start the main loop again, if we are in a worker.
   if (_globalState.isWorker) return;