Introduce some basic utility classes.

Analyzer improvements

One might argue, the most basic utility classes.

- strengthen analyzer against error messages that point past the
  bottom of the file.

- ignore any errors in intl's auto-generated files.

Automatically fill in the menu button when there's a drawer

Add support for letterSpacing to TextStyle

Also, fill in a back arrow when we can go back.

Fixes #699

Workaround logcat -c not working

Be more discerning with forward transitions

Allow non-Material apps to use the Title widget.

Navigator.canPop(context)

So you don't fade out when going to a popup menu, for example.

Make it possible to specify the saved path for trace --stop

@chinmaygarde

RenderFlex shouldn't assert when its out of space

Let MaterialApp.onGenerateRoute return a Route

Add menu items to stocks to control timeDilation

Previously, we would assert with FlexJustifyContent.collapse if we ran out of
space. Now we do the same thing we do for the other types of justification,
which is to let the children's layout overflow.

Also:
 - minor code reindents in places.
 - reset the widget tree between tests.
 - once you generate a route, don't let its builder change
   (previously it would keep changing as the routes table changed).
 - revert the stocks app toolbar-fading-on-forward-transition thing.

Add support for passing --trace-startup on Android

iOS takes a different codepath and will need more work.

@chinmaygarde

Support `flutter test` with prebuilt binaries

Fix a large number of Drawer bugs

This patch restructures how we handle drawer. The drawer is now a child of the
Scaffold, which wraps the Drawer in a DrawerController. The DrawerController
manages the interaction with the navigator as well as the edge swiping. The
DrawerController's state machine is driven almost entirely off its Performance,
which it now owns completely.

Fixes #90
Fixes #187
Fixes #192
Fixes #194
Fixes #604

Now you can run tests with and without --flutter-repo with a prebuilt binary on
Linux.

Fixes #307

buildForwardTransition()

For those times when you want to do something as you move away from a
route into the next one, as well as when you move into it from the
previous one.

Rename some of the functions from the scheduler.

The names are probably less familiar, but more consistent:
- FrameCallback: a callback that is relative to the frame and wants the
  frame offset (a duration) as argument.
- addXFrameCallback: adds the given callback to the internal lists/maps.
- scheduleXFrameCallback (currently only X = ""): add the callback, but
  also trigger a new frame.
- handleX: the method that is invoked when the event-loop or the frame
  calls into the scheduler.
- ensureXYZ: ensure that the callback happens.
  Unfortunately there is the ambiguity between a "callback": it can be a
  closure, or the action of doing a callback, so we end up with:
  ensureBeginFrameCallback, and ensureEventLoopCallback, where
  "callback" means the action of being called back.

Add missing import to flutter_sprites

Add liftToOverlay to Mimicable

This patch integrates Mimicable with Overlay such that you can tell a Mimicable
to lift up into the overlay and animate towards another widget identified by a
global key.

Fix a typo in _CircularProgressIndicatorPainter