Merge NNBD branch into master (flutter#270)

[pointer_interceptor] Migrate RaisedButton to ElevatedButton (flutter…

…#266)

RaisedButton was deprecated recently

[pointer_interceptor] Fix README for pub.dev (flutter#258)

Tweak the README.md slightly so it renders better in pub.dev (which seems to be completely removing the center tag).

[web] Add PointerInterceptor widget (flutter#256)

The widget can be used as a wrapper of any widget that is overlaid on top of a platform view, so it can handle clicks "as expected".

More info about usage and testing in the corresponding READMEs!

Prepare animations for 1.1.2 (flutter#183)

[animations] Handle DualTransitionBuilder name conflict with flutter/…

…flutter (flutter#173)

* Handle DualTransitionBuilder conflict with flutter/flutter

Require animations package to use internal version of DualTransitionBuilder to avoid conflicts with flutter/flutter\'s recently merged copy of DualTransitionBuilder'.

* Update changelog and pubspec.yaml

[animations] Prepare for 1.1.0 (flutter#165)

* Update changelog and pubspec for new release

[animations] Prepare for 1.1.0 (flutter#165)

* Update changelog and pubspec for new release

[animations] Fix analyzer ignore location (flutter#110)

* Fix analyzer ignore location

[animations] Fix analyzer warnings (flutter#109)

* Update pubspec.yaml

* Update CHANGELOG.md

* Fix analyzer warnings