Reland https://code.google.com/p/dart/source/detail?r=29315:

runtime/vm/isolate.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "lib/mirrors.h"
@@ skipping 516 matching lines @@
     // communicate with the spawner to receive the initial message before it
     // executes the real entry point.
     // Since this function ("RunIsolate") is used for both Isolate.spawn and
     // Isolate.spawnUri we also send a boolean flag as argument so that the
     // "_startIsolate" function can act corresponding to how the isolate was
     // created.
     const Array& args = Array::Handle(Array::New(2));
     args.SetAt(0, Instance::Handle(func.ImplicitStaticClosure()));
     args.SetAt(1, is_spawn_uri ? Bool::True() : Bool::False());
 
+    // Dispatching through _startIsolate will open a control port as a live
+    // port. Account for this by increasing the number of open control ports.
+    isolate->message_handler()->increment_control_ports();
+
     const Library& lib = Library::Handle(Library::IsolateLibrary());
     const String& entry_name = String::Handle(String::New("_startIsolate"));
     const Function& entry_point =
         Function::Handle(lib.LookupLocalFunction(entry_name));
     ASSERT(entry_point.IsFunction() && !entry_point.IsNull());
 
     result = DartEntry::InvokeFunction(entry_point, args);
     if (result.IsError()) {
       StoreError(isolate, result);
       return false;
@@ skipping 510 matching lines @@
   return func.raw();
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(isolate());
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart