constructor tear-offs language feature specification must be accepted

[jcollins-g]

Per dart-lang/sdk#46234 (comment), for implementation to begin on many issues in dart-lang/sdk#46228 the constructor tear-offs language feature specification must be accepted.

[lrhn]

The tear-off feature has been accepted. The current design document is now in the /accepted folder.

The designated experiment name is (as predicted) "constructor-tearoffs".