[async-iteration] expose Symbol.asyncIterator behind flag --harmony-async-iteration

Flag is used by followup patches split apart from
https://codereview.chromium.org/2622833002/, and tests for each split
out CL ends up using Symbol.asyncIterator, so it makes sense to land it
first (behind a flag).

BUG=v8:5855
R=littledan@chromium.org, adamk@chromium.org

Review-Url: https://codereview.chromium.org/2645923003
Cr-Commit-Position: refs/heads/master@{#42527}

src/bootstrapper.cc

@@ -3592,6 +3592,13 @@ void Genesis::InitializeGlobal_harmony_array_prototype_values() {
                         NONE);
 }
 
+void Genesis::InitializeGlobal_harmony_async_iteration() {
+  if (!FLAG_harmony_async_iteration) return;
+  Handle<JSFunction> symbol_fun(native_context()->symbol_function());
+  InstallConstant(isolate(), symbol_fun, "asyncIterator",
+                  factory()->async_iterator_symbol());
+}
+
 Handle<JSFunction> Genesis::InstallArrayBuffer(Handle<JSObject> target,
                                                const char* name,
                                                Builtins::Name call,
@@ -4057,6 +4064,7 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_trailing_commas_natives[] = {nullptr};
   static const char* harmony_class_fields_natives[] = {nullptr};
   static const char* harmony_object_rest_spread_natives[] = {nullptr};
+  static const char* harmony_async_iteration_natives[] = {nullptr};
 
   for (int i = ExperimentalNatives::GetDebuggerCount();
        i < ExperimentalNatives::GetBuiltinsCount(); i++) {

src/flag-definitions.h

@@ -202,7 +202,8 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
   V(harmony_regexp_named_captures, "harmony regexp named captures")      \
   V(harmony_regexp_property, "harmony unicode regexp property classes")  \
   V(harmony_class_fields, "harmony public fields in class literals")     \
-  V(harmony_object_rest_spread, "harmony object rest spread properties")
+  V(harmony_object_rest_spread, "harmony object rest spread properties") \
+  V(harmony_async_iteration, "harmony async iteration")
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                                   \

src/heap-symbols.h

@@ -243,6 +243,7 @@
   V(uninitialized_symbol)
 
 #define PUBLIC_SYMBOL_LIST(V)                    \
+  V(async_iterator_symbol, Symbol.asyncIterator) \
   V(iterator_symbol, Symbol.iterator)            \
   V(intl_fallback_symbol, IntlFallback)          \
   V(match_symbol, Symbol.match)                  \

test/mjsunit/harmony/symbol-async-iterator.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-async-iteration
+
+assertTrue(Symbol.hasOwnProperty('asyncIterator'));
+assertEquals('symbol', typeof Symbol.asyncIterator);
+assertInstanceof(Object(Symbol.asyncIterator), Symbol);
+
+let desc = Object.getOwnPropertyDescriptor(Symbol, 'asyncIterator');
+assertFalse(desc.writable);
+assertFalse(desc.enumerable);
+assertFalse(desc.configurable);