Make defaultRouteName immutable.

And trivial docs changes.

Provide an API for hashCode getters.

Remove old 'back' callback, and add dartdocs.

This adds some dartdocs to dart:ui, specifically for Window and friends.

Mojo wrapper for the Android SoundPool API

Improve FontWeigh and TextDecoration

Instead of using enums, these are now real classes, which lets us add `bold`
and `normal` alias to FontWeight and a `combine` constructor to TextDecoration.

Update Dart dependency to include fixes in x64 Dart precompilation

Require the user to specify the DART_SDK_PATH manually in Xcode

Fix mouse on linux.

Fix crash with empty rect

Don't attempt to draw with a zero-by-zero window.

Revving engine

Fix pointers in mojo.

Make the mojo implementation of the pointer packet logic fake move
events when it would send ups or downs without corresponding moves.

Turn on FLX updater.

Improve the crash fix in rasterizer_mojo

The previous version can still crash if we try to draw between the time when we
initialize context_ and when we actually make the MGL context current.

Don't crash when RasterizerMojo::Draw happens without a context

If we don't have a context, we can't draw, so bail out early.

Rev pub packages

Tell Flutter about the host platform's locale

Don't copy mojo keyboard service now that they have diverged.

Related to #299.

Make RRect useful

Other changes:
 - Remove the default constructor for RRect and Rect.
 - Add trailing periods to Offset comments.
 - Add Offset.distanceSquared for people who want to compare x^2+y^2
   to r^2 without paying the cost of the sqrt. (Corresponding fix to
   OvalRect coming in a separate flutter patch.)
 - Fix a bug in Rect.lerp().
 - Fix various Rect comments.
 - Remove a "var" in Rect.

Fixes #769.

Add a bunch of missing Path functions

Part of #252