Add unit tests to flattenStyle

Summary: The StyleSheet merging algorithm was modeled after `Object.assign` and the native spread operator. This diff converts `flattenStyle` to actually use `Object.assign`.
Closes https://github.com/facebook/react-native/pull/3048

Reviewed By: @​svcscm

Differential Revision: D2506387

Pulled By: @vjeux

Libraries/StyleSheet/__tests__/flattenStyle-test.js

@@ -0,0 +1,52 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+jest.autoMockOff();
+
+var flattenStyle = require('flattenStyle');
+
+describe('flattenStyle', () => {
+
+  it('should merge style objects', () => {
+    var style1 = {width: 10};
+    var style2 = {height: 20};
+    var flatStyle = flattenStyle([style1, style2]);
+    expect(flatStyle.width).toBe(10);
+    expect(flatStyle.height).toBe(20);
+  });
+
+  it('should override style properties', () => {
+    var style1 = {backgroundColor: '#000', width: 10};
+    var style2 = {backgroundColor: '#023c69', width: null};
+    var flatStyle = flattenStyle([style1, style2]);
+    expect(flatStyle.backgroundColor).toBe('#023c69');
+    expect(flatStyle.width).toBe(null);
+  });
+
+  it('should overwrite properties with `undefined`', () => {
+    var style1 = {backgroundColor: '#000'};
+    var style2 = {backgroundColor: undefined};
+    var flatStyle = flattenStyle([style1, style2]);
+    expect(flatStyle.backgroundColor).toBe(undefined);
+  });
+
+  it('should not fail on falsy values', () => {
+    expect(() => flattenStyle([null, false, undefined])).not.toThrow();
+  });
+
+  it('should recursively flatten arrays', () => {
+    var style1 = {width: 10};
+    var style2 = {height: 20};
+    var style3 = {width: 30};
+    var flatStyle = flattenStyle([null, [], [style1, style2], style3]);
+    expect(flatStyle.width).toBe(30);
+    expect(flatStyle.height).toBe(20);
+  });
+});