Fix race in Debug_InterruptIsolate which results in the following crash:

runtime/vm/debugger_api_impl_test.cc

 // Copyright (c) 2012, 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 "include/dart_mirrors_api.h"
 #include "include/dart_tools_api.h"
 #include "platform/assert.h"
 #include "vm/dart_api_impl.h"
 #include "vm/lockers.h"
 #include "vm/unit_test.h"
@@ skipping 1493 matching lines @@
   EXPECT_VALID(result);
 
   Dart_Handle retval = Invoke("main");
   EXPECT_VALID(retval);
   Dart_ExitScope();
   Dart_ShutdownIsolate();
 }
 
 
 TEST_CASE(Debug_InterruptIsolate) {
+  sync = new Monitor();
   Dart_SetIsolateEventHandler(&TestInterruptIsolate);
-  sync = new Monitor();
   EXPECT(interrupt_isolate_id == ILLEGAL_ISOLATE_ID);
   Dart_SetPausedEventHandler(InterruptIsolateHandler);
   int result = OSThread::Start("DebugInterruptIsolate", InterruptIsolateRun, 0);
   EXPECT_EQ(0, result);
 
   // Wait for the test isolate to be created.
   {
     MonitorLocker ml(sync);
     while (interrupt_isolate_id == ILLEGAL_ISOLATE_ID) {
       ml.Wait();
@@ skipping 795 matching lines @@
       " null, 3, 7, 1, 8, 6, 9, 10, 10, 11, 11, 13,"
       " null, 4, 13, 3, 14, 10,"
       " null, 5, 17, 5, 18, 9, 19, 12,"
       " null, 6, 21, 1,"
       " null, 8, 24, 1, 25, 5, 26, 6, 27, 8,"
       " null, 9, 29, 1]",
       tokens_cstr);
 }
 
 }  // namespace dart