Cleanup dangling comment references.

Quick pass at fixing a few dangling references as revealed by the new `comment_references` lint (https://github.com/dart-lang/linter/issues/240).

There's a bunch more to do here before we can turn it on by default (~430 lints as of now).  Many of them are a simple matter of adding an import (e.g., `dart:async` for library docs that reference `Future`) but others will require a bit of thought.  Probably best done by the folks writing the code. :)

Don't suggest filing an issue when we can definitively say that the
assertion in question was not thrown from within the flutter package.
Fixes https://github.com/flutter/flutter/issues/3812.

Put the stack trace first after the message, with more details below the
stack trace, since the stack is often very useful and the stacks are no
longer stupidly long.

Better document the 'rescheduling' feature, and improve the error
handling asserts when it is misused.
Fixes https://github.com/flutter/flutter/issues/3888.

Improve the wording around the stack dump for exceptions during
callbacks.

Improve the color and font size of messages in live tests.
Fixes https://github.com/flutter/flutter/issues/4018.

This patch updates the date picker to match the new spec.

We're still missing a fade effect when scrolling from one month to
another and we're missing the landscape layout.

Fixes #3558

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? ;)