Update to new analyzer and linter.

Notably, this will allow us to play with

* the fixed `public_member_api_docs` that now checks for documented getters when checking setters (https://github.com/dart-lang/linter/issues/237), and
* the new `comment_references` lint that ensures identifiers referenced in docs are in scope (https://github.com/dart-lang/linter/issues/240).

This reverts commit f91cf1c4.

Notably, this will allow us to play with

* the fixed `public_member_api_docs` that now checks for documented getters when checking setters (https://github.com/dart-lang/linter/issues/237), and
* the new `comment_references` lint that ensures identifiers referenced in docs are in scope (https://github.com/dart-lang/linter/issues/240).

This moves all the bot-related files to `dev/bots`, hiding it from our
home page in github. Also, simplifies the travis setup, though that
doesn't do any difference to the performance sadly.

The latest engine roll caused a performance regression: https://github.com/flutter/flutter/issues/4005

Also, clean up the DatePicker to use more modern technology, such as a
grid for displaying the days of the month.

Fixes #3976

Also, make some previously private classes public for better
documentation.

* 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).

Also, remove several unused configuration options and fix an animation
leak.

Also, make sure that broken tests actually break the bots.

And add a test to make sure that keeps happening.

This also fixes some related problems affecting "flutter run":
  * FLXes built during AndroidDevice.startApp need to match the build mode
  * APKs should always be rebuilt if the build mode uses AOT compilation

BUG=https://github.com/flutter/flutter/issues/3970

* ScrollableBuilder tweaks

We now required the curve parameter for CurvedAnimation.

Re-enable `implementation_imports` lint rule.

Also, add some missing docs to http.dart and widgets.dart.

Everything now has dartdocs except one setter whose getter already has
docs.

Back out SDK summaries.

Remove redundant type annotation lint.

Make flutter run generate identical apks to flutter build apk

* Add a Scrollable builder, refactor ScrollableList, et al

* Add space between the dialog demo buttons

* removed vestigial code

According to the profile for the flow manual test, we're spending the vast
majority of our time recording timeline traces. This patch removes the timeline
traces, which greatly improves performance.

Also, optimize TransformLayer to avoid one matrix memcpy. I filed
https://github.com/google/vector_math.dart/issues/166 about an API that would
make this even faster.

SInce this is a subset of `always_specify_types` I think we can safely remove it.

In practice, if you violate you get doubly nagged:
```
[lint] Type annotate public APIs. (packages/flutter_tools/lib/src/dart/runner.dart, line 5, col 1)
[lint] Declare method return types. (packages/flutter_tools/lib/src/dart/runner.dart, line 5, col 1)
```

One warning is probably enough to get the message across? ;)

Net-net: SDK summaries are not safe in the presence of SDK library embedders.

Context (and bug to track re-enablement) here: https://github.com/dart-lang/sdk/issues/26467.

This exposes the default throttling implementation, and an alternative
non-throttling implementation, of `debugPrint`.

* gracefully handle generating docs twice

* silently handle if directory doesn't exist

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.

Remove the unimplemented `more` option from the copy/paste toolbar.

Punting this feature for the near time, so I'm removing the dead option
from the toolbar.