Using anchor links from https://github.com/flutter/website/pull/676

Update to reflect https://github.com/flutter/flutter/pull/11751 and https://github.com/flutter/website/pull/663

* Flutter create widget test template.

Running `flutter create —with-widget-test` produces a test/widget_test.dart sample widget test.

* Generate widget test bits in flutter create.

* Path fix.

* Added types.

* Skip shell testing on windows.

* formatting fixes

* Update test sample to test the sample app.

* Formatting tweak.

Plugin projects are created by running `flutter create --plugin <name>`.

An example app is also created in the plugin project, using the normal 'create' template, which has been modified to allow for conditional plugin code.

Modified the android package name to match package naming conventions (all lower-case, and must match the directory name).

Switch our pubspec.yamls to using SDK sources so that we can have consistent
source types when we depend on these packages from external packages using SDK
sources.