Also use the word "dropdown" consistently over "drop down".

The theme fallback mechanism was busted by the shadowThemeOnly change.

Fixes #6562

When I changed how routes complete their futures, I broke the Dropdown
button because it was still waiting for its own Completer to complete
instead of using the Future returned by push. This patch fixes that
issue.

I've also removed the previous behavior of the DropdownButton forwarding
its text style to its route. The mechansim that we were using doesn't
work properly in all cases. For example, if the DropdownButton is a
child of a LayoutBuilder, then the route will have already built by the
time the DropdownButton gets a chance to forward its text style, causing
an assert in setState.

Finally, I've tweaked PopupMenuButton to work the same way as
DropdownButton in a couple corner cases (e.g., not calling the changed
callback if the button was removed from the tree before the menu
completed its Future).

Fixes #6352

Fixes #3208.

* Add a "build" phase to EnginePhase for completeness.
* Ignore events from the device during test execution.
* More dartdocs
* Slightly more helpful messages about Timers in verifyInvariants.
* Add widgetList, elementList, stateList, renderObjectList.
* Send test events asynchronously for consistency with other APIs.
* Fix a test that was depending on test events being synchronous (or
  rather, scheduled in a microtask that came before the microtask for
  the completer of the future that the tap() function returned).

This makes it possible to substitute 'flutter run' for 'flutter test'
and actually watch a test run on a device.

For any test that depends on flutter_test:

1. Remove any import of 'package:test/test.dart'.

2. Replace `testWidgets('...', (WidgetTester tester) {`
      with `testWidgets('...', (WidgetTester tester) async {`

3. Add an "await" in front of calls to any of the following:
    * tap()
    * tapAt()
    * fling()
    * flingFrom()
    * scroll()
    * scrollAt()
    * pump()
    * pumpWidget()

4. Replace any calls to `tester.flushMicrotasks()` with calls to
   `await tester.idle()`.

There's a guarding API that you can use, if you have particularly
complicated tests, to get better error messages. Search for
TestAsyncUtils.

Previously it used Block, which is less efficient for large numbers of items.
Also move the top margin out of the menu item to fix the baseline alignment of
the text.

Fixes #1615

This patch sizes the menu such that it is always on screen, but doesn't scroll
the menu to ensure that the currently selected item is always visible and on
top of the button. That will need to wait for a later patch.

Also, teach CustomPaint how to repaint animations more efficiently.

Fixes #3720