Add Array#flat non-extensible test

test/built-ins/Array/prototype/flat/target-array-non-extensible.js

@@ -0,0 +1,48 @@
+// 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: >
+  TypeError is thrown if CreateDataProperty fails.
+  (result object is non-extensible, source array is not 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
+        [...]
+      vi. Else,
+        [...]
+        2. Perform ? CreateDataPropertyOrThrow(target, ! ToString(targetIndex), element).
+      [...]
+
+  CreateDataPropertyOrThrow ( O, P, V )
+
+  [...]
+  3. Let success be ? CreateDataProperty(O, P, V).
+  4. If success is false, throw a TypeError exception.
+features: [Symbol.species]
+---*/
+
+var A = function(_length) {
+  this.length = 0;
+  Object.preventExtensions(this);
+};
+
+var arr = [1];
+arr.constructor = {};
+arr.constructor[Symbol.species] = A;
+
+assert.throws(TypeError, function() {
+  arr.flat(1);
+});