flutter build --engine-src-dir=<garbage> produces poor error message

This patch improves the error message.

Fixes #468

Route refactor

- Removed the concept of ephemeral routes.
- Renamed the two _MenuRoutes to _PopupMenuRoute and _DropDownRoute.
- Added type arguments in various places:
  - DropDownMenu
  - _DropDownRoute
  - _ModalBottomSheetRoute
  - PopupMenuItem
  - _PopupMenu
  - _PopupMenuRoute
- Made _ModalBottomSheetRoute, the two ex _MenuRoutes, and _DialogRoute
  all inherit from ModalRoute, via PopupRoute.
- Change "Dropdown" and "DropDown" to "DropDown" consistently.
- Made MaterialPageRoute inherit from PageRoute.
- Made ModalBarrier not create a box if it's always transparent.
- Exposed the Futures on TransitionRoutes.
- Fixed that menus were no longer dismissable by tapping the modal
  barrier.

Give type arguments to routes.

Rename LabeledGlobalKey's argument to debugLabel

Fix analyzer warnings in updater package

These end up not actually being used, currently, because we don't have
generic methods, which you'd need for showDialog() and friends, and we
don't have any way to parameterise a class type at runtime, which you'd
need for MaterialApp routes, but it's a step in the right direction.

The old name was confusing.

Fixes #462

This class was renamed to MojoEventSubscription.

Be more specific about what `flutter build` does

first addition of _embedder.yaml

OverlayRoute.finished()

Rather than have delayed calls to super.didPop(), which raises my
eyebrow every time I see it, this provides a separate finished()
function to call, and uses the convention that if you want to call it
yourself, you just don't call super.didPop().

Elaborate HSVColor interface

Fixes #507

Expose the close()/closed API for snack bars

This lets you close them manually.

Convert Persistent Bottom Sheets to a Scaffold API

- `Scaffold.of(context).showBottomSheet(widget);`
- Returns an object with .closed Future and .close() method.
- Uses a StateRoute to handle back button.
- Take the Navigator logic out of the BottomSheet widget.
- Support showing a sheet while an old one is going away.
- Add Navigator.remove().

Download build artifacts from a zip file

Make stocks list scrolling 2x faster on the main thread

This updates the Flutter tools to match the proposed new packaging of artifacts
in the engine release script.
* The GCS URL for artifacts is now gs://mojo/flutter/$revision/$platform
* Categories have been removed from the Artifact class
* All artifacts for a given platform now live in a zip file.  If an artifact
  is not present in the local cache, then the zip will be downloaded and
  extracted.

Note that darwin-x64 artifacts go through a different process that (for now)
continues to use the old format.

This patch improves the repaint strategy for HomogeneousViewport so that the
list itself and every entry in the list has a repaint boundary. That means that
we only record list items as they enter the view.

Fixes #512

Callback identity is too fragile for CustomPaint

Many of the widgets that use CustomPaint were spamming repaints because
CustomPaint repaints when the identity of the onPaint callback changes, which
it does every build for StatelessComponents.

This patch changes CustomPaint to use a CustomPainter, similar to the new
custom layout widgets. The CustomPainter has a `shouldRepaint` function along
with its `paint` function. This function gives clients explicit control over
when the custom paint object repaints.

Add a FractionalOffset alignment parameter to BackgroundImage

Add a repaint rainbow to help debug repaint issues

The repaint raindbow tints each recording with a rotating set of colors so you
can see which pixels are repainting each frame.

Add debugDumpLayerTree to dump the layer tree

To help debugging issues with the layer tree.

Re-enable support for debugPaintLayerBordersEnabled

Removes demo game from repository (it's now in its own repo)

We lost this feature at some point. This patch brings it back.

Introduced ImageProvider for asynchronously loading images.