[async-iteration] implement Async-from-Sync Iterator

src/runtime/runtime-internal.cc

 // Copyright 2014 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.
 
 #include "src/runtime/runtime-utils.h"
 
 #include <memory>
 
 #include "src/arguments.h"
 #include "src/ast/prettyprinter.h"
@@ skipping 487 matching lines @@
 RUNTIME_FUNCTION(Runtime_AllowDynamicFunction) {
   HandleScope scope(isolate);
   DCHECK_EQ(1, args.length());
   CONVERT_ARG_HANDLE_CHECKED(JSFunction, target, 0);
   Handle<JSObject> global_proxy(target->global_proxy(), isolate);
   return *isolate->factory()->ToBoolean(
       Builtins::AllowDynamicFunction(isolate, target, global_proxy));
 }
 
 RUNTIME_FUNCTION(Runtime_CreateAsyncFromSyncIterator) {
-  // TODO(caitp): split AsyncFromSyncIterator functionality out of
-  //              https://codereview.chromium.org/2622833002
-  UNREACHABLE();
-  return isolate->heap()->undefined_value();
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+
+  CONVERT_ARG_HANDLE_CHECKED(Object, sync_iterator, 0);
+
+  if (!sync_iterator->IsJSReceiver()) {
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kSymbolIteratorInvalid));
+  }
+
+  return *isolate->factory()->NewJSAsyncFromSyncIterator(
+      Handle<HeapObject>::cast(sync_iterator));
 }
 
 }  // namespace internal
 }  // namespace v8