* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

* work around const lints

* update nonconst doc

...and other minor Border improvements.
And tests.

This changes the merge logic I added yesterday to not support nulls
but instead support BorderSide.none and equivalents. This makes more
sense when dealing with actual Borders.