Fix race in Debug_InterruptIsolate which results in the following crash:

runtime/vm/debugger_api_impl_test.cc

Index: runtime/vm/debugger_api_impl_test.cc
diff --git a/runtime/vm/debugger_api_impl_test.cc b/runtime/vm/debugger_api_impl_test.cc
index cd1b8c817bab26813406e9b83afc1d5ca4821308..84878012636be6b4b6dbd673443f28dd5328b77e 100644
--- a/runtime/vm/debugger_api_impl_test.cc
+++ b/runtime/vm/debugger_api_impl_test.cc
@@ -1511,8 +1511,8 @@ static void InterruptIsolateRun(uword unused) {
 
 
 TEST_CASE(Debug_InterruptIsolate) {
-  Dart_SetIsolateEventHandler(&TestInterruptIsolate);
   sync = new Monitor();
+  Dart_SetIsolateEventHandler(&TestInterruptIsolate);
   EXPECT(interrupt_isolate_id == ILLEGAL_ISOLATE_ID);
   Dart_SetPausedEventHandler(InterruptIsolateHandler);
   int result = OSThread::Start("DebugInterruptIsolate", InterruptIsolateRun, 0);