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

runtime/vm/port.cc

Index: runtime/vm/port.cc
===================================================================
--- runtime/vm/port.cc	(revision 1762)
+++ runtime/vm/port.cc	(working copy)
@@ -90,7 +90,6 @@
 
 Dart_Port PortMap::CreatePort() {
   Isolate* isolate = Isolate::Current();
-
   MutexLocker ml(mutex_);
 
   Entry entry;
@@ -162,6 +161,9 @@
 
 void PortMap::ClosePorts() {
   Isolate* isolate = Isolate::Current();
+  if (isolate->active_ports() == 0) {
+    return;
+  }
   {
     MutexLocker ml(mutex_);
     for (intptr_t i = 0; i < capacity_; i++) {