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 1762)
+++ runtime/vm/isolate.h	(working copy)
@@ -75,6 +75,12 @@
     active_ports_--;
   }
 
+  Dart_Port main_port() { return main_port_; }
+  void set_main_port(Dart_Port port) {
+    ASSERT(main_port_ == 0);  // Only set main port once.
+    main_port_ = port;
+  }
+
   Heap* heap() const { return heap_; }
   void set_heap(Heap* value) { heap_ = value; }
   static intptr_t heap_offset() { return OFFSET_OF(Isolate, heap_); }
@@ -241,6 +247,7 @@
   Dart_PostMessageCallback post_message_callback_;
   Dart_ClosePortCallback close_port_callback_;
   intptr_t active_ports_;
+  Dart_Port main_port_;
   Heap* heap_;
   ObjectStore* object_store_;
   StackResource* top_resource_;