Extension for RecordType doesn't auto-complete fields without this.

[FMorschel]

Consider this code:

extension on ({int foo, int bar}) {
  int get baz => foo + bar;
  int get qux => this.foo + this.bar;
}

Without this. in front, no completion is shown for foo or bar, even though they are in scope and are valid.

Also, the colouring is off on VS Code (CC @DanTup):

[FMorschel]

I suppose that colouring will fix itself if the first issue is fixed, but it is probably best to verify that.

I'll see what I can do about the completion first.

[FMorschel]

Here is the CL https://dart-review.googlesource.com/c/sdk/+/424684

[FMorschel]

Here is the second CL for fixing the colouring https://dart-review.googlesource.com/c/sdk/+/426342.