[new feature] add overlay component

src/overlay/test/__snapshots__/demo.spec.js.snap

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+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`renders demo correctly 1`] = `
+<div>
+  <div><button class="van-button van-button--primary van-button--normal" style="margin-left: 15px;"><span class="van-button__text">显示遮罩层</span></button>
+    <div class="van-overlay" style="z-index: 1; display: none;" name="van-fade"></div>
+  </div>
+</div>
+`;

src/overlay/test/__snapshots__/index.spec.js.snap

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+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`class-name prop 1`] = `<div class="van-overlay my-overlay" style="z-index: 1;" name="van-fade"></div>`;
+
+exports[`duration prop 1`] = `<div class="van-overlay" style="z-index: 1; animation-duration: 1s;" name="van-fade"></div>`;
+
+exports[`z-index prop 1`] = `<div class="van-overlay" style="z-index: 99;" name="van-fade"></div>`;

src/overlay/test/demo.spec.js

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+import Demo from '../demo';
+import demoTest from '../../../test/demo-test';
+
+demoTest(Demo);

src/overlay/test/index.spec.js

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+import { mount } from '../../../test/utils';
+import Overlay from '..';
+
+test('z-index prop', () => {
+  const wrapper = mount(Overlay, {
+    propsData: {
+      show: true,
+      zIndex: 99
+    }
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});
+
+test('class-name prop', () => {
+  const wrapper = mount(Overlay, {
+    propsData: {
+      show: true,
+      className: 'my-overlay'
+    }
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});
+
+test('duration prop', () => {
+  const wrapper = mount(Overlay, {
+    propsData: {
+      show: true,
+      duration: 1
+    }
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});
+
+test('click event', () => {
+  const onClick = jest.fn();
+  const wrapper = mount(Overlay, {
+    context: {
+      on: {
+        click: onClick
+      }
+    }
+  });
+
+  wrapper.trigger('click');
+  expect(onClick).toHaveBeenCalledTimes(1);
+});