Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant. (#7944)

* Add find.descendant API to support find descendants of an element by passing the current element and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

* Add find.descendant API to support find descendants of an element by passing the current element finder and the finder of descendant.

This patch contains some basic tests for PaginatedDataTable.

Add an option to provide a custom description to predicate
finders. Without a custom description we default to printing the
predicate function's signature, which is not all that useful.

Use this new option in the driver extension to print the text of the
sought after tooltip.

This issue got fixed by a recent check-in, but I had a test specifically
for this incarnation so we should probably keep it just in case.

Closes https://github.com/flutter/flutter/issues/5630

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.

* Refactor widget test framework

Instead of:

```dart
  test("Card Collection smoke test", () {
    testWidgets((WidgetTester tester) {
```

...you now say:

```dart
  testWidgets("Card Collection smoke test", (WidgetTester tester) {
```

Instead of:

```dart
  expect(tester, hasWidget(find.text('hello')));
```

...you now say:

```dart
  expect(find.text('hello'), findsOneWidget);
```

Instead of the previous API (exists, widgets, widget, stateOf,
elementOf, etc), you now have the following comprehensive API. All these
are functions that take a Finder, except the all* properties.

* `any()` - true if anything matches, c.f. `Iterable.any`
* `allWidgets` - all the widgets in the tree
* `widget()` - the one and only widget that matches the finder
* `firstWidget()` - the first widget that matches the finder
* `allElements` - all the elements in the tree
* `element()` - the one and only element that matches the finder
* `firstElement()` - the first element that matches the finder
* `allStates` - all the `State`s in the tree
* `state()` - the one and only state that matches the finder
* `firstState()` - the first state that matches the finder
* `allRenderObjects` - all the render objects in the tree
* `renderObject()` - the one and only render object that matches the finder
* `firstRenderObject()` - the first render object that matches the finder

There's also `layers' which returns the list of current layers.

`tap`, `fling`, getCenter, getSize, etc, take Finders, like the APIs
above, and expect there to only be one matching widget.

The finders are:

 * `find.text(String text)`
 * `find.widgetWithText(Type widgetType, String text)`
 * `find.byKey(Key key)`
 * `find.byType(Type type)`
 * `find.byElementType(Type type)`
 * `find.byConfig(Widget config)`
 * `find.byWidgetPredicate(WidgetPredicate predicate)`
 * `find.byElementPredicate(ElementPredicate predicate)`

The matchers (for `expect`) are:

 * `findsNothing`
 * `findsWidgets`
 * `findsOneWidget`
 * `findsNWidgets(n)`
 * `isOnStage`
 * `isOffStage`
 * `isInCard`
 * `isNotInCard`

Benchmarks now use benchmarkWidgets instead of testWidgets.

Also, for those of you using mockers, `serviceMocker` now automatically
handles the binding initialization.

This patch also:

* changes how tests are run so that we can more easily swap the logic
  out for a "real" mode instead of FakeAsync.

* introduces CachingIterable.

* changes how flutter_driver interacts with the widget tree to use the
  aforementioned new API rather than ElementTreeTester, which is gone.

* removes ElementTreeTester.

* changes the semantics of a test for scrollables because we couldn't
  convince ourselves that the old semantics made sense; it only worked
  before because flushing the microtasks after every event was broken.

* fixes the flushing of microtasks after every event.

* Reindent the tests

* Fix review comments