Reapply 0489249d29131ed7bf047485c74f557d88eb9974 with a fix for front_end tests.

runtime/lib/stacktrace.cc

Index: runtime/lib/stacktrace.cc
diff --git a/runtime/lib/stacktrace.cc b/runtime/lib/stacktrace.cc
index d8b8cab176a7f68d71890d1f3cba74d00aedc45a..c5c41f2573d9f4c6e9cd0624e959e60ada688cbc 100644
--- a/runtime/lib/stacktrace.cc
+++ b/runtime/lib/stacktrace.cc
@@ -107,13 +107,15 @@ DEFINE_NATIVE_ENTRY(StackTrace_current, 0) {
 }
 
 DEFINE_NATIVE_ENTRY(StackTrace_asyncStackTraceHelper, 1) {
+  if (!FLAG_causal_async_stacks) {
+    return Object::null();
+  }
+
   GET_NATIVE_ARGUMENT(Closure, async_op, arguments->NativeArgAt(0));
-  if (!async_op.IsNull()) {
-    if (FLAG_support_debugger) {
-      Debugger* debugger = isolate->debugger();
-      if (debugger != NULL) {
-        debugger->MaybeAsyncStepInto(async_op);
-      }
+  if (FLAG_support_debugger) {
+    Debugger* debugger = isolate->debugger();
+    if (debugger != NULL) {
+      debugger->MaybeAsyncStepInto(async_op);
     }
   }
   return CurrentStackTrace(thread, true);
@@ -125,6 +127,10 @@ DEFINE_NATIVE_ENTRY(StackTrace_clearAsyncThreadStackTrace, 0) {
 }
 
 DEFINE_NATIVE_ENTRY(StackTrace_setAsyncThreadStackTrace, 1) {
+  if (!FLAG_causal_async_stacks) {
+    return Object::null();
+  }
+
   GET_NON_NULL_NATIVE_ARGUMENT(StackTrace, stack_trace,
                                arguments->NativeArgAt(0));
   thread->set_async_stack_trace(stack_trace);