[CFE] Evaluate constants inside patterns

[alexmarkov]

Part of #49749.

main() {
  List value = const [42];
  if (value case const [42]) {
    print('OK');
  } else {
    print('FAIL');
  }
}

Kernel:

   core::List<dynamic> value = #C2;
    final core::List<dynamic> #t1 = value;
    final core::bool #t2 = true;
    if(#t1 =={core::List::==}{(core::Object) → core::bool} core::_GrowableList::_literal1<dynamic>(42)) {
      #t2 = false;
      {
        core::print("OK");
      }
    }
    if(#t2) {
      core::print("FAIL");
    }

The first const [42] was evaluated to a constant #C2, but the 2nd const [42] (in the pattern if (value case const [42])) was not evaluated. This results in runtime failures in certain tests (such as co19/LanguageFeatures/Patterns/constant_A03_t02, co19/LanguageFeatures/Patterns/constant_A03_t04 etc).

/cc @johnniwinther

[johnniwinther]

@stereotype441 This happens because the handleLiteralList call for the const [42] in value case const [42] doesn't have a constKeyword. The const keyword is instead provided to the handleConstantPattern method.

I've created https://dart-review.googlesource.com/c/sdk/+/273960 which instead includes the const keyword as part of the subexpression, thereby normalizing the handling of expression that can be prefixed by const.

[stereotype441]

@bwilkerson what do you think of Johnni's AST changes in https://dart-review.googlesource.com/c/sdk/+/273960? They seem reasonable to me, but since you designed the analyzer's AST representation for patterns I want to get your input.