tests for String.prototype.matchAll changes (#2389)

Per https://github.com/tc39/ecma262/pull/1716

test/built-ins/String/prototype/matchAll/flags-nonglobal-throws.js

@@ -0,0 +1,42 @@
+// Copyright (C) 2019 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: pending
+description: Re-throws errors when calling @@matchAll
+info: |
+  String.prototype.matchAll ( regexp )
+    [...]
+    2. If _regexp_ is neither *undefined* nor *null*, then
+             1. Let _isRegExp_ be ? IsRegExp(_regexp_).
+             1. If _isRegExp_ is true, then
+               1. Let _flags_ be ? Get(_regexp_, *"flags"*).
+               1. Perform ? RequireObjectCoercible(_flags_).
+               1. If ? ToString(_flags_) does not contain *"g"*, throw a *TypeError* exception.
+features: [Symbol.matchAll]
+---*/
+
+
+assert.throws(TypeError, function () {
+  ''.matchAll(/a/);
+});
+assert.throws(TypeError, function () {
+  ''.matchAll(/a/i);
+});
+assert.throws(TypeError, function () {
+  ''.matchAll(/a/m);
+});
+assert.throws(TypeError, function () {
+  ''.matchAll(/a/u);
+});
+assert.throws(TypeError, function () {
+  ''.matchAll(/a/y);
+});
+
+var regex = /a/;
+Object.defineProperty(regex, 'flags', {
+  value: 'muyi'
+});
+
+assert.throws(TypeError, function () {
+  ''.matchAll(regex);
+});

test/built-ins/String/prototype/matchAll/flags-undefined-throws.js

@@ -0,0 +1,33 @@
+// Copyright (C) 2019 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: pending
+description: Re-throws errors when calling @@matchAll
+info: |
+  String.prototype.matchAll ( regexp )
+    [...]
+    2. If _regexp_ is neither *undefined* nor *null*, then
+             1. Let _isRegExp_ be ? IsRegExp(_regexp_).
+             1. If _isRegExp_ is true, then
+               1. Let _flags_ be ? Get(_regexp_, *"flags"*).
+               1. Perform ? RequireObjectCoercible(_flags_).
+               1. If ? ToString(_flags_) does not contain *"g"*, throw a *TypeError* exception.
+features: [Symbol.matchAll]
+---*/
+
+var regex = /a/g;
+Object.defineProperty(regex, 'flags', { value: undefined });
+
+assert.throws(TypeError, function () {
+  ''.matchAll(regex);
+});
+
+Object.defineProperty(RegExp.prototype, 'flags', {
+  get: function () {
+    return undefined;
+  }
+});
+
+assert.throws(Test262Error, function () {
+  ''.matchAll(/a/g);
+});