Implement constructor tearoffs support in language tests and in co19 tests

[eernstg]

This issue tracks support of the constructor tearoffs feature. See the enclosing meta-issue for details.

The work on these tests has started:

• co19 tests
• language tests

[devoncarew]

@eernstg - is this item done (or if not, who's the best owner?)?

[eernstg]

The co19 tests are here: https://github.com/dart-lang/co19/tree/master/LanguageFeatures/Constructor-tear-offs.

I'm working on https://dart-review.googlesource.com/c/sdk/+/211140 which adds a test about constant factory contructors #46984, and I'm about to add a test about syntactic disambiguation, dart-lang/language#1806.

@lrhn, do you have any upcoming constructor-tearoffs tests? Current language tests are here:

typedef/aliased_type_literal_instantiation_test.dart
typedef/aliased_constructor_tear_off_test.dart
constructor/reference_test.dart
constructor/tear_off_test.dart
constructor/unnamed_new_error_test.dart
constructor/unnamed_new_test.dart
generic_methods/explicit_instantiated_tearoff_test.dart
type_object/explicit_instantiated_type_literal_error_test.dart
type_object/explicit_instantiated_type_literal_test.dart

So we're nearly done. I've assigned myself as owner.

[eernstg]

No further proposals, so I'll close this one.