Dropdown: onChange ev.target now always consistent (#8538)

* Updating onChange of Dropdown to have a consistent target.

* change file.

* Update change file.

common/changes/office-ui-fabric-react/dropdown-onchange_2019-03-29-18-15.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "office-ui-fabric-react",
+      "comment": "Dropdown: The `onChange` callback's event target should always be the dropdown element, which contains the id attribute passed in through props.",
+      "type": "patch"
+    }
+  ],
+  "packageName": "office-ui-fabric-react",
+  "email": "[email protected]"
+}

packages/office-ui-fabric-react/src/components/Dropdown/Dropdown.base.tsx

@@ -330,7 +330,8 @@ export class DropdownBase extends BaseComponent<IDropdownInternalProps, IDropdow
       // for single-select, option passed in will always be selected.
       // for multi-select, flip the checked value
       const changedOpt = multiSelect ? { ...options[index], selected: !checked } : options[index];
-      onChange(event, changedOpt, index);
+
+      onChange({ ...event, target: this._dropDown.current as EventTarget }, changedOpt, index);
     }
 
     if (onChanged) {

packages/office-ui-fabric-react/src/components/Dropdown/Dropdown.test.tsx

@@ -162,7 +162,10 @@ describe('Dropdown', () => {
       const onChangeSpy = jest.fn();
 
       try {
-        ReactDOM.render(<Dropdown label="testgroup" defaultSelectedKey="1" onChange={onChangeSpy} options={DEFAULT_OPTIONS} />, container);
+        ReactDOM.render(
+          <Dropdown id="foo" label="testgroup" defaultSelectedKey="1" onChange={onChangeSpy} options={DEFAULT_OPTIONS} />,
+          container
+        );
         dropdownRoot = container.querySelector('.ms-Dropdown') as HTMLElement;
 
         ReactTestUtils.Simulate.click(dropdownRoot);
@@ -171,6 +174,7 @@ describe('Dropdown', () => {
         ReactTestUtils.Simulate.click(secondItemElement);
       } finally {
         expect(onChangeSpy).toHaveBeenCalledWith(expect.anything(), DEFAULT_OPTIONS[2], 2);
+        expect(onChangeSpy.mock.calls[0][0].target.id).toEqual('foo');
       }
     });