Add a mid-sized integration test for the Dart Embedding Api which

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 1518)
+++ runtime/vm/isolate.h	(working copy)
@@ -46,6 +46,13 @@
 
   StoreBufferBlock* store_buffer() { return &store_buffer_; }
 
+  Dart_CreatePortCallback create_port_callback() const {
+    return create_port_callback_;
+  }
+  void set_create_port_callback(Dart_CreatePortCallback value) {
+    create_port_callback_ = value;
+  }
+
   Dart_PostMessageCallback post_message_callback() const {
     return post_message_callback_;
   }
@@ -238,6 +245,7 @@
   StoreBufferBlock store_buffer_;
   Monitor* monitor_;
   MessageQueue* message_queue_;
+  Dart_CreatePortCallback create_port_callback_;
   Dart_PostMessageCallback post_message_callback_;
   Dart_ClosePortCallback close_port_callback_;
   intptr_t active_ports_;