Array.prototype.flatten => Array.prototype.flat (#1569)

features.txt

test/built-ins/Array/prototype/flat/array-like-objects.js

test/built-ins/Array/prototype/flat/bound-function-call.js

test/built-ins/Array/prototype/flat/empty-array-elements.js

test/built-ins/Array/prototype/flat/empty-object-elements.js

test/built-ins/Array/prototype/flat/length.js

test/built-ins/Array/prototype/flat/name.js

...rray/prototype/flat/non-numeric-depth-should-not-throw.js

.../built-ins/Array/prototype/flat/non-object-ctor-throws.js

...built-ins/Array/prototype/flat/null-undefined-elements.js

...t-ins/Array/prototype/flat/null-undefined-input-throws.js

test/built-ins/Array/prototype/flat/positive-infinity.js

test/built-ins/Array/prototype/flat/prop-desc.js

.../prototype/flat/symbol-object-create-null-depth-throws.js