[dart 4.0] Remove support for `new` in constructor invocations

[munificent]

We made new optional in Dart 2.0, 8 years ago. We have a lint that tells people not to use new. The lint is in the recommended set. Our core library and Flutter docs don't use new.

It has never meant anything semantic because even an expression marked new might be invoking a factory constructor which doesn't allocate an object.

How about we finally get rid of it as a keyword at constructor invocations completely?

[srawlins]

Alternatively, could this be done in Dart 3.x?

[lrhn]

could this be done in Dart 3.x?

Technically, yes.

We usually do feature removal on major version increments. People can stop using the feature any time they want. 😁