This updates to mojo 4e4d51ce28a and mojo sdk 711a0bcfb141b4 and updates the sky
package's pubspec.yaml dependency to '>=0.1.0 <0.2.0' to be compatible with
the current mojo package. This includes an update to the Mojo Dart generator to
produce real classes for enums and the corresponding updates for users of the
KeyboardType enum in Sky as well as one scoped_ptr->std::unique_ptr in shell
corresponding to a change in the Mojo EDK.
When a new version of the sky and sky_services package are pushed this will fix
domokit/mojo#440.
This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.
These widgets were designed in CSS where you need to specify a layout model for
your children. This patch updates them to the modern style of just taking a
unique child.
Fixes#755
