Type parameter highlight doesn't trigger on declaration

[FMorschel]

Repro:

/// This is a comment that references [ThisType]
extension <ThisType> on ThisType {
  ThisType foo() {
    return this;
  }
}

/// This is a comment that references [ThisType]
extension type Et<ThisType>(ThisType value) {
  ThisType get v => value;
}

/// This is a comment that references [ThisType]
mixin M<ThisType> {
  ThisType get t;
}

/// This is a comment that references [ThisType]
abstract class C<ThisType> {
  ThisType get t;
}

/// This is a comment that references [ThisType]
enum E<ThisType> {
  a;

  ThisType get t => throw UnimplementedError();
}

/// This is a comment that references [ThisType]
void foo<ThisType>() {
  ThisType? t;
}

[FMorschel]

Extensions are also never highlighted, I'll fix this together.

[FMorschel]

Here is the CL https://dart-review.googlesource.com/c/sdk/+/419423