[Records] CFE rejects record constants with multiple named fields

[alexmarkov]

Part of #49713.

Example:

void main() {
  print(const (foo: 1, bar: 2));
}

CFE issues the following errors:

foo.dart:2:21: Error: Constant evaluation error:
  print(const (foo: 1, bar: 2));
                    ^
foo.dart:2:21: Context: The variable '(unnamed)' is not a constant, only constant expressions are allowed.
  print(const (foo: 1, bar: 2));
                    ^

The following constants work fine:

  print(const (1, 2));
  print(const (foo: 1));

/cc @johnniwinther