Add a Async Select that fetches options from given endpoint (#1909)

* Add a Async Select that fetches options from given endpoint

* update it statement

superset/assets/spec/javascripts/components/AsyncSelect_spec.jsx

@@ -0,0 +1,35 @@
+import React from 'react';
+import Select from 'react-select';
+import AsyncSelect from '../../../javascripts/components/AsyncSelect';
+import { shallow } from 'enzyme';
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+import sinon from 'sinon';
+
+describe('AsyncSelect', () => {
+  const mockedProps = {
+    dataEndpoint: '/slicemodelview/api/read',
+    onChange: sinon.spy(),
+    mutator: () => {},
+  };
+  it('is valid element', () => {
+    expect(
+      React.isValidElement(<AsyncSelect {...mockedProps} />)
+    ).to.equal(true);
+  });
+
+  it('has one select', () => {
+    const wrapper = shallow(
+      <AsyncSelect {...mockedProps} />
+    );
+    expect(wrapper.find(Select)).to.have.length(1);
+  });
+
+  it('calls onChange on select change', () => {
+    const wrapper = shallow(
+      <AsyncSelect {...mockedProps} />
+    );
+    wrapper.find(Select).simulate('change', { value: 1 });
+    expect(mockedProps.onChange).to.have.property('callCount', 1);
+  });
+});