Add Array#flat non-writable property test

test/built-ins/Array/prototype/flat/target-array-with-non-writable-property.js

@@ -0,0 +1,50 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-array.prototype.flat
+description: >
+  Non-writable properties are overwritten by CreateDataProperty.
+  (result object's "0" is non-writable, source array gets flattened)
+info: |
+  Array.prototype.flat ( [ depth ] )
+
+  [...]
+  6. Perform ? FlattenIntoArray(A, O, sourceLen, 0, depthNum).
+
+  FlattenIntoArray ( target, source, sourceLen, start, depth [ , mapperFunction, thisArg ] )
+
+  [...]
+  9. Repeat, while sourceIndex < sourceLen
+    [...]
+    c. If exists is true, then
+      [...]
+      v. If shouldFlatten is true, then
+        [...]
+        2. Set targetIndex to ? FlattenIntoArray(target, element, elementLen, targetIndex, depth - 1).
+      [...]
+features: [Symbol.species]
+includes: [propertyHelper.js]
+---*/
+
+var A = function(_length) {
+  Object.defineProperty(this, "0", {
+    value: 1,
+    writable: false,
+    enumerable: false,
+    configurable: true,
+  });
+};
+
+var arr = [[2]];
+arr.constructor = {};
+arr.constructor[Symbol.species] = A;
+
+var res = arr.flat(1);
+
+verifyProperty(res, "0", {
+  value: 2,
+  writable: true,
+  enumerable: true,
+  configurable: true,
+});