Add new tests about i2b and super-bounded types

language/generic/super_bounded_types_and_variance_test.dart
is based on pkg/front_end/testcases/top_level_variance_test.dart,
but updated such that it expects the two cases using `source9` to
succeed (due to relaxed rules for what it takes to be super-bounded).

language/generic/super_bounded_types_and_nested_variance_test.dart is
similarly based on
pkg/front_end/testcases/general/nested_variance_test.dart,

language/generic/super_bounded_types_test.dart tests that a selection of
correctly super-bounded types are accepted.

language/generic/super_bounded_types_error_test.dart tests that a
selection of types which are not correctly super-bounded are
reported as errors.

Change-Id: If1b9bf432e0218739bd6efd5abac52878d679f14
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/158397
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>