Implement is-checks against type variables.

tests/language/language_dart2js.status

Index: tests/language/language_dart2js.status
diff --git a/tests/language/language_dart2js.status b/tests/language/language_dart2js.status
index c36d5577908adf83a6d3ff942d78674a0af76705..5e7c6f44fd476ad02e5b863cd82d75cbaf598b44 100644
--- a/tests/language/language_dart2js.status
+++ b/tests/language/language_dart2js.status
@@ -32,6 +32,8 @@ class_literal_test/29: Fail # Class literals are expression now; delete this tes
 *vm_negative_test: Skip
 
 [ $compiler == dart2js && $checked ]
+mixin_type_parameters_super_extends_test: Fail # http://dartbug.com/8116
+mixin_type_parameters_super_test: Fail # http://dartbug.com/8116
 prefix16_test: Fail # dartbug.com/7354
 default_factory2_test/01: Pass # For the wrong reasons.
 type_variable_scope_test/none: Fail
@@ -202,9 +204,7 @@ on_catch_malformed_type_test: Fail # Malformed types cause compile-time errors.
 
 # Failing tests for mixin type parameters.
 mixin_type_parameters_mixin_test: Fail # http://dartbug.com/8116
-mixin_type_parameters_super_test: Fail # http://dartbug.com/8116
 mixin_type_parameters_mixin_extends_test: Fail # http://dartbug.com/8116
-mixin_type_parameters_super_extends_test: Fail # http://dartbug.com/8116
 
 
 # External tests.