Add initial test files from lodash v4. (#4172)

* Install test dependencies.

* Add initial test files.

These files were created using a simplistic AST manipulator using `recast` to
preserve formatting. There's bound to be a huge chunk of errors, but this serves
as a good start. QUnit was replaced with Mocha, with ES2015 imports running via
`esm`.

As far as possible, QUnit-specific syntax has been replaced with Mocha's
`describe` and `it`, while the native Node.js `assert` module is used for
assertions. Files in the `test` directory ending in `.test.js` will be treated
as test files.

* Add initial passing files to test run.

test/flatten-methods.js

@@ -0,0 +1,106 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { args, _ } from './utils.js';
+import flatten from '../flatten.js';
+import flattenDeep from '../flattenDeep.js';
+import flattenDepth from '../flattenDepth.js';
+
+describe('flatten methods', function() {
+  var array = [1, [2, [3, [4]], 5]],
+      methodNames = ['flatten', 'flattenDeep', 'flattenDepth'];
+
+  it('should flatten `arguments` objects', function() {
+    var array = [args, [args]];
+
+    assert.deepStrictEqual(flatten(array), [1, 2, 3, args]);
+    assert.deepStrictEqual(flattenDeep(array), [1, 2, 3, 1, 2, 3]);
+    assert.deepStrictEqual(flattenDepth(array, 2), [1, 2, 3, 1, 2, 3]);
+  });
+
+  it('should treat sparse arrays as dense', function() {
+    var array = [[1, 2, 3], Array(3)],
+        expected = [1, 2, 3];
+
+    expected.push(undefined, undefined, undefined);
+
+    lodashStable.each(methodNames, function(methodName) {
+      var actual = _[methodName](array);
+      assert.deepStrictEqual(actual, expected);
+      assert.ok('4' in actual);
+    });
+  });
+
+  it('should flatten objects with a truthy `Symbol.isConcatSpreadable` value', function() {
+    if (Symbol && Symbol.isConcatSpreadable) {
+      var object = { '0': 'a', 'length': 1 },
+          array = [object],
+          expected = lodashStable.map(methodNames, lodashStable.constant(['a']));
+
+      object[Symbol.isConcatSpreadable] = true;
+
+      var actual = lodashStable.map(methodNames, function(methodName) {
+        return _[methodName](array);
+      });
+
+      assert.deepStrictEqual(actual, expected);
+    }
+  });
+
+  it('should work with extremely large arrays', function() {
+    lodashStable.times(3, function(index) {
+      var expected = Array(5e5);
+      try {
+        var func = flatten;
+        if (index == 1) {
+          func = flattenDeep;
+        } else if (index == 2) {
+          func = flattenDepth;
+        }
+        assert.deepStrictEqual(func([expected]), expected);
+      } catch (e) {
+        assert.ok(false, e.message);
+      }
+    });
+  });
+
+  it('should work with empty arrays', function() {
+    var array = [[], [[]], [[], [[[]]]]];
+
+    assert.deepStrictEqual(flatten(array), [[], [], [[[]]]]);
+    assert.deepStrictEqual(flattenDeep(array), []);
+    assert.deepStrictEqual(flattenDepth(array, 2), [[[]]]);
+  });
+
+  it('should support flattening of nested arrays', function() {
+    assert.deepStrictEqual(flatten(array), [1, 2, [3, [4]], 5]);
+    assert.deepStrictEqual(flattenDeep(array), [1, 2, 3, 4, 5]);
+    assert.deepStrictEqual(flattenDepth(array, 2), [1, 2, 3, [4], 5]);
+  });
+
+  it('should return an empty array for non array-like objects', function() {
+    var expected = [],
+        nonArray = { '0': 'a' };
+
+    assert.deepStrictEqual(flatten(nonArray), expected);
+    assert.deepStrictEqual(flattenDeep(nonArray), expected);
+    assert.deepStrictEqual(flattenDepth(nonArray, 2), expected);
+  });
+
+  it('should return a wrapped value when chaining', function() {
+    var wrapped = _(array),
+        actual = wrapped.flatten();
+
+    assert.ok(actual instanceof _);
+    assert.deepEqual(actual.value(), [1, 2, [3, [4]], 5]);
+
+    actual = wrapped.flattenDeep();
+
+    assert.ok(actual instanceof _);
+    assert.deepEqual(actual.value(), [1, 2, 3, 4, 5]);
+
+    actual = wrapped.flattenDepth(2);
+
+    assert.ok(actual instanceof _);
+    assert.deepEqual(actual.value(), [1, 2, 3, [4], 5]);
+  });
+});