Update documentation to cover constructor tearoffs

[eernstg]

This issue tracks support of the constructor tearoffs feature. See the enclosing meta-issue for details.

[devoncarew]

cc @kwalrath

[leafpetersen]

Does this cover any effective dart work required? cc @munificent

[jacob314]

It would align well with lints if effective dart explained why you should get used to always using tearoffs when they have the same result.
E.g.
Never write:
foo.forEach((e) => print(e))
when you can write
for.forEach(print)
for all kinds of tearoffs.

[munificent]

I think we will want to reword the guideline around tear-offs a little since the current wording is limited to methods. Filed dart-lang/site-www#3560 to track.

[devoncarew]

pinging this issue; @kwalrath, we expect to have the feature complete for the branch at the start of Nov., and shipping in the Dec release.