Documentation changes, and, in particular, the ability to lerp from
and to any decoration by lerping via null.

FlutterLogoDecoration should defer to Decoration for what to do when
it doesn't know how to lerp to/from another kind of decoration.

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

Covers FlutterLogoDecoration.lerp()

Disabling const constructor lint for the test values pending resolution of
https://github.com/dart-lang/sdk/issues/26980