Add parser support for use of `new` as a constructor name.

The new "constructor tearoffs" feature includes the ability to use the
`new` keyword in a place where an identifier is expected, and that
identifier could denote a constructor name.  We handle this by
replacing the `new` keyword token with an identifier token whose
identifier string is `new` (this should ease the burden on the
implementations, since they are already set up to handle constructor
names that are identifiers).

Since all such situations follow a `.`, and `new` was never previously
allowed after a `.`, the parser treats `new` as an identifier in any
situation where it could possibly refer to a constructor, regardless
of whether the "constructor tearoffs" feature is enabled.  (This
should allow for easier error recovery in the situation where the user
tries to use the feature with a language feature that does not support
it).  It is up to the client to report an error if the feature is
disabled.

In this CL, I've implemented logic for the analyzer to choose whether
to report an error based on whether the feature is enabled.  I've
implemented logic for the CFE to report the error unconditionally.

Bug: https://github.com/dart-lang/sdk/issues/46020, https://github.com/dart-lang/sdk/issues/46044
Change-Id: I36a496688400d2d9f699dd42be4d0ba620cda244
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201961
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>