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

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
===================================================================
--- runtime/vm/isolate.cc	(revision 1762)
+++ runtime/vm/isolate.cc	(working copy)
@@ -37,6 +37,7 @@
       post_message_callback_(NULL),
       close_port_callback_(NULL),
       active_ports_(0),
+      main_port_(0),
       heap_(NULL),
       object_store_(NULL),
       top_resource_(NULL),
@@ -125,6 +126,7 @@
   // TODO(5411455): Need to figure out how to set the stack limit for the
   // main thread.
   result->SetStackLimitFromCurrentTOS(reinterpret_cast<uword>(&result));
+  result->set_main_port(PortMap::CreatePort());
 
   return result;
 }
@@ -241,7 +243,7 @@
   ASSERT(post_message_callback() == &StandardPostMessageCallback);
   ASSERT(close_port_callback() == &StandardClosePortCallback);
 
-  while (active_ports() > 0) {
+  while (Dart_IsolateHasActivePorts()) {
     ASSERT(this == Isolate::Current());
     Zone zone(this);
     HandleScope handle_scope(this);