This patch completes the circuit on this library. The next step is to go
through and add docs to anything we missed.

* ClipPath

* Add a test for ClipOval and ClipPath

We were looking for this service in the wrong service provider.

Fixes #3683

Support for putting assets in android/assets folder (like ios/Resources)

* Track the worst frame in perf tests

* Use backticks in "/// Returns `null` if"

This makes it easier to generate charts that show the engine time and
the framework time separately.

Also, move the default theme transition duration into theme.dart.

* LayoutBuilder Widget

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

Add note about explicit `pub get` needed.

Fixes https://github.com/flutter/flutter/issues/3493

 - rebuild stateless widgets that have dependencies when their ancestors change but they don't

Fixes https://github.com/flutter/flutter/issues/3120

 - rebuild widgets that tried to inherit from a widget that didn't exist, when the widget is added

This adds a pointer and a bool to Element, which isn't great. It also adds a more or less complete tree walk when you add a new Inherited widget at the top of your tree, which isn't cheap.

@devoncarew @jason-simmons

If there's a max height or width, we should factor that into the intrinsic
sizing for the other dimension.

We need to import these enums in order for the dartdoc to linkify correctly.

Fixes #3668

Check the adb install output for errors and stop "flutter run" if package installation fails (#3672)

Also update the engine to a build where these tools have been uplaoded

Fixes #3624

* Revert "Revert "Mega gallery (#3647)" (#3659)"

This reverts commit ea62d31f.

* fix the dev/dartdoc.dart script

This reverts commit 9497fb56.

This patch broke the dartdoc step on the bots.

Pick up a bug fix to the `usage` package.

* working on a large benchmark app

* build a 100k saple app

* print sizes of other libs; scale to 60k lines

* test the generated app on travis

Instead of incorporating the margin into the button, introduce a ButtonBar
widget that supplies the proper spacing between the buttons. Also, make these
buttons more configurable via ButtonTheme so that dialogs can change the
minWidth and padding of the buttons as required by the spec.

Fixes #1843
Fixes #3184

Also add dartdocs.

* Initial set of tests for flutter sprites

This currently requires a local build of the engine.

Run "flutter build aot" to build an AOT snapshot.
Run "flutter build apk --profile" to build a "profile mode" APK that uses AOT.

* 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

These wrappers also give us a chance to document what these services do.

Fixes #3164

Turns out there were more clients of the old paragraph API than I expected.
This patch migrates them to the new API.