Sdk: Editing assists add new/const even when run in dart 2 mode

[mit-mit]

Repro steps:

1. Enable dart 2 preview in the editor
2. Use the editing assist to 'Center widget'

Expected result: assist adds Center(child: ...)
Actual result: assist adds new Center(child: ...)

[mit-mit]

cc @scheglov

[devoncarew]

We should likely try and match the surrounding code style here, or key off a user preference or analysis options file setting?

[mit-mit]

I'd be much bolder and always omit these when using Dart 2 for Flutter. I see no reason why we could encourage manually adding new/const for Flutter code.

[mit-mit]

(We can always add a setting later if there is lots of demand for it)

[devoncarew]

From some discussion, a setting available in the analysis options file, and falling back to the --preview-dart-2 state, would likely hit satisfy most use cases.

[devoncarew]

We may delay implementing an analysis options setting until we know there's need for it.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/42320