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/isPlainObject.js

@@ -0,0 +1,114 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+
+import {
+  document,
+  create,
+  objectProto,
+  falsey,
+  stubFalse,
+  symbol,
+  defineProperty,
+  realm,
+} from './utils.js';
+
+import isPlainObject from '../isPlainObject.js';
+
+describe('isPlainObject', function() {
+  var element = document && document.createElement('div');
+
+  it('should detect plain objects', function() {
+    function Foo(a) {
+      this.a = 1;
+    }
+
+    assert.strictEqual(isPlainObject({}), true);
+    assert.strictEqual(isPlainObject({ 'a': 1 }), true);
+    assert.strictEqual(isPlainObject({ 'constructor': Foo }), true);
+    assert.strictEqual(isPlainObject([1, 2, 3]), false);
+    assert.strictEqual(isPlainObject(new Foo(1)), false);
+  });
+
+  it('should return `true` for objects with a `[[Prototype]]` of `null`', function() {
+    var object = create(null);
+    assert.strictEqual(isPlainObject(object), true);
+
+    object.constructor = objectProto.constructor;
+    assert.strictEqual(isPlainObject(object), true);
+  });
+
+  it('should return `true` for objects with a `valueOf` property', function() {
+    assert.strictEqual(isPlainObject({ 'valueOf': 0 }), true);
+  });
+
+  it('should return `true` for objects with a writable `Symbol.toStringTag` property', function() {
+    if (Symbol && Symbol.toStringTag) {
+      var object = {};
+      object[Symbol.toStringTag] = 'X';
+
+      assert.deepStrictEqual(isPlainObject(object), true);
+    }
+  });
+
+  it('should return `false` for objects with a custom `[[Prototype]]`', function() {
+    var object = create({ 'a': 1 });
+    assert.strictEqual(isPlainObject(object), false);
+  });
+
+  it('should return `false` for DOM elements', function() {
+    if (element) {
+      assert.strictEqual(isPlainObject(element), false);
+    }
+  });
+
+  it('should return `false` for non-Object objects', function() {
+    assert.strictEqual(isPlainObject(arguments), false);
+    assert.strictEqual(isPlainObject(Error), false);
+    assert.strictEqual(isPlainObject(Math), false);
+  });
+
+  it('should return `false` for non-objects', function() {
+    var expected = lodashStable.map(falsey, stubFalse);
+
+    var actual = lodashStable.map(falsey, function(value, index) {
+      return index ? isPlainObject(value) : isPlainObject();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+
+    assert.strictEqual(isPlainObject(true), false);
+    assert.strictEqual(isPlainObject('a'), false);
+    assert.strictEqual(isPlainObject(symbol), false);
+  });
+
+  it('should return `false` for objects with a read-only `Symbol.toStringTag` property', function() {
+    if (Symbol && Symbol.toStringTag) {
+      var object = {};
+      defineProperty(object, Symbol.toStringTag, {
+        'configurable': true,
+        'enumerable': false,
+        'writable': false,
+        'value': 'X'
+      });
+
+      assert.deepStrictEqual(isPlainObject(object), false);
+    }
+  });
+
+  it('should not mutate `value`', function() {
+    if (Symbol && Symbol.toStringTag) {
+      var proto = {};
+      proto[Symbol.toStringTag] = undefined;
+      var object = create(proto);
+
+      assert.strictEqual(isPlainObject(object), false);
+      assert.ok(!lodashStable.has(object, Symbol.toStringTag));
+    }
+  });
+
+  it('should work with objects from another realm', function() {
+    if (realm.object) {
+      assert.strictEqual(isPlainObject(realm.object), true);
+    }
+  });
+});