Fix causal_async_star_stack_contents_test with optimization-counter-threshold=5

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
diff --git a/runtime/vm/debugger.cc b/runtime/vm/debugger.cc
index 0c0edcc769aa9480232142a1447d2ae444b78668..a74a0f53fb38beae9814b94fdaa9e8c8f4466b7e 100644
--- a/runtime/vm/debugger.cc
+++ b/runtime/vm/debugger.cc
@@ -1680,7 +1680,7 @@ DebuggerStackTrace* Debugger::CollectAsyncCausalStackTrace() {
   // asynchronous dispatch mechanisms.
   StackFrameIterator iterator(false);
   StackFrame* frame = iterator.NextFrame();
-  while (synchronous_stack_trace_length >= 0) {
+  while (synchronous_stack_trace_length > 0) {
     ASSERT(frame != NULL);
     if (frame->IsDartFrame()) {
       code = frame->LookupDartCode();