Add an early error case for ({ async async })

test/language/expressions/object/prop-def-invalid-async-prefix.js

+// Copyright (C) 2017 Mozilla Corporation. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: >
+  async is not a valid prefix of an identifier reference
+esid: sec-object-initializer
+info: |
+    PropertyDefinition:
+      IdentifierReference
+      CoverInitializedName
+      PropertyName : AssignmentExpression
+      MethodDefinition
+
+    MethodDefinition:
+      PropertyName ( UniqueFormalParameters ) { FunctionBody }
+      AsyncMethod
+
+    AsyncMethod:
+      async [no LineTerminator here] PropertyName ( UniqueFormalParameters ) { AsyncFunctionBody }
+      VariableDeclaration : BindingPattern Initializer
+
+      1. Let rhs be the result of evaluating Initializer.
+      2. Let rval be GetValue(rhs).
+      3. ReturnIfAbrupt(rval).
+      4. Return the result of performing BindingInitialization for
+         BindingPattern passing rval and undefined as arguments.
+negative:
+  phase: early
+  type: SyntaxError
+---*/
+
+({async async});