isolates refactor: this change introduces 'dart:isolate' as a library. This is a

lib/isolate/runtime/ports.dart

Index: lib/isolate/runtime/ports.dart
diff --git a/runtime/lib/isolate.dart b/lib/isolate/runtime/ports.dart
similarity index 85%
rename from runtime/lib/isolate.dart
rename to lib/isolate/runtime/ports.dart
index 13ec8dd66cd9f39bfcfb6878fbeea9f35e76e278..7d74fa20ec49737710fd1dbc398b1695e10ef494 100644
--- a/runtime/lib/isolate.dart
+++ b/lib/isolate/runtime/ports.dart
@@ -140,18 +140,3 @@ class SendPortImpl implements SendPort {
 }
 
 
-class IsolateNatives {
-  static Future<SendPort> spawn(Isolate isolate, bool isLight) {
-    Completer<SendPort> completer = new Completer<SendPort>();
-    SendPort port = _start(isolate, isLight);
-    completer.complete(port);
-    return completer.future;
-  }
-
-  // Starts a new isolate calling the run method on a new instance of the
-  // remote class's type.
-  // Returns the send port which is passed to the newly created isolate.
-  // This method is being dispatched to from the public core library code.
-  static SendPort _start(Isolate isolate, bool light)
-      native "IsolateNatives_start";
-}