WIP [esnext] implement async iteration proposal

test/test262/local-tests/test/language/expressions/async-generators/early-errors-expression-binding-identifier-eval.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+author: Caitlin Potter <caitp@igalia.com>
+esid: pending
+description: >
+  If the source code matching this production is strict code, it is a
+  Syntax Error if BindingIdentifier is the IdentifierName eval.
+negative:
+  phase: early
+  type: SyntaxError
+flags: [onlyStrict]
+---*/
+
+(async function* eval() { });