WIP [esnext] implement async iteration proposal

test/test262/local-tests/test/language/expressions/async-generators/early-errors-expression-formals-contains-yield.js

Index: test/test262/local-tests/test/language/expressions/async-generators/early-errors-expression-formals-contains-yield.js
diff --git a/test/test262/local-tests/test/language/expressions/async-generators/early-errors-expression-formals-contains-yield.js b/test/test262/local-tests/test/language/expressions/async-generators/early-errors-expression-formals-contains-yield.js
new file mode 100644
index 0000000000000000000000000000000000000000..cbc96a8836012d5f3aa8ce1660f1241f79c4c2bc
--- /dev/null
+++ b/test/test262/local-tests/test/language/expressions/async-generators/early-errors-expression-formals-contains-yield.js
@@ -0,0 +1,15 @@
+// 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: 12.1
+description: >
+  `await` is a reserved keyword within async generator function bodies and may
+  not be used as the binding identifier of a parameter.
+negative:
+  phase: early
+  type: SyntaxError
+---*/
+
+(async function*(yield) { });