[parser] don't report error for CoverInitializedNames in async arrow formals

test/mjsunit/harmony/async-await-basic.js

 // Copyright 2016 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-await --allow-natives-syntax
 
 // Do not install `AsyncFunction` constructor on global object
 
 function assertThrowsAsync(run, errorType, message) {
   var actual;
@@ skipping 325 matching lines @@
 log = [];
 assertEqualsAsync(6, () => resumeAfterThrowNewEval(5));
 assertEquals("start:5 resume:throw1 resume:throw2", log.join(" "));
 
 async function foo() {}
 assertEquals("async function foo() {}", foo.toString());
 assertEquals("async function () {}", async function() {}.toString());
 assertEquals("async x => x", (async x => x).toString());
 assertEquals("async x => { return x }", (async x => { return x }).toString());
 class AsyncMethod { async foo() { } }
-assertEquals("async foo() { }", Function.prototype.toString.call(AsyncMethod.prototype.foo));
-assertEquals("async foo() { }", Function.prototype.toString.call({async foo() { }}.foo));
+assertEquals("async foo() { }",
+             Function.prototype.toString.call(AsyncMethod.prototype.foo));
+assertEquals("async foo() { }",
+             Function.prototype.toString.call({async foo() { }}.foo));
 
 // Async functions are not constructible
 assertThrows(() => class extends (async function() {}) {}, TypeError);
+
+// Regress v8:5148
+assertEqualsAsync("1", () => (async({ a = NaN }) => a)({ a: "1" }));
+assertEqualsAsync(
+    "10", () => (async(foo, { a = NaN }) => foo + a)("1", { a: "0" }));
+assertEqualsAsync("2", () => (async({ a = "2" }) => a)({ a: undefined }));
+assertEqualsAsync(
+    "20", () => (async(foo, { a = "0" }) => foo + a)("2", { a: undefined }));
+assertThrows(() => eval("async({ foo = 1 })"), SyntaxError);
+assertThrows(() => eval("async(a, { foo = 1 })"), SyntaxError);