Run JS micro tasks in the appropriate context.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index a4b311f6ecacd7d4b4a760931475bf4d9405c2ec..a387c20b28cc964ec2ec02895f7eb4ae9e7bbdb7 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2292,12 +2292,10 @@ void Isolate::EnqueueMicrotask(Handle<Object> microtask) {
 
 
 void Isolate::RunMicrotasks() {
-  // TODO(adamk): This ASSERT triggers in mjsunit tests which
-  // call the %RunMicrotasks runtime function. But it should
-  // never happen outside of tests, so it would be nice to
-  // uncomment it.
-  //
-  // ASSERT(handle_scope_implementer()->CallDepthIsZero());
+  // In some mjsunit tests %RunMicrotasks is called explicitly, violating
+  // this assertion.  Therefore we also check for --allow-natives-syntax.
+  ASSERT(FLAG_allow_natives_syntax ||
+         handle_scope_implementer()->CallDepthIsZero());
 
   // Increase call depth to prevent recursive callbacks.
   v8::Isolate::SuppressMicrotaskExecutionScope suppress(
@@ -2317,6 +2315,8 @@ void Isolate::RunMicrotasks() {
       if (microtask->IsJSFunction()) {
         Handle<JSFunction> microtask_function =
             Handle<JSFunction>::cast(microtask);
+        SaveContext save(this);
+        set_context(microtask_function->context()->native_context());
         Handle<Object> exception;
         MaybeHandle<Object> result = Execution::TryCall(
             microtask_function, factory()->undefined_value(),