Miscellaneous dartdoc markdown fixes

Widgets App fixes and minor newton cleanup

Add a bool to indicate whether we're in MojoShell

More dart docs for text painter

Add Decoration.lerp and document Decoration

- Make the textStyle optional
- Remove routes
- Make onGenerateRoute required
- Add some dartdocs

Update engine.version

Fix position of the overlay

Blocked on https://github.com/flutter/flutter/pull/2771

Now that the overlay actually respects the position we give it, put it
somewhere where the keyboard won't overlap it.

Add some docs for people looking at bindings

Improve Stack documentation.

Fixed some typos.

handle android n in the android sdk

Use Canvas#drawParagraph

Material: Fix selected tab indicator animation.

Sniff service protocol port from device

- The indicator no longer goes crazy when you select another tab while
  it's animating.
- The indicator's trailing edge lags behind the trailing edge, giving it
  a neat stretchy feel.

BUG=https://github.com/flutter/flutter/issues/1617
BUG=https://github.com/flutter/flutter/issues/1955

We'll eventually remove Paragraph#paint.

Fixes #2694

Update engine

update listen to work w/ ios devices

In MojoShell, we can connect to other applications. This patch adds a bool that
clients can use to check whether that feature is available without needing to
try to connect to an application.

show failures with the initial pub get

Refactor the build commands

Fix spelling of dependents

Noticed in #2733