dart2js: Fix some problems with mixins and generics

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 b6e331a14d354dfe4f0109e34ae9a07ce20836b5..55566a3756975746c4c4f138712db04d95919af2 100644
--- a/tests/language/language_dart2js.status
+++ b/tests/language/language_dart2js.status
@@ -17,8 +17,6 @@ try_catch_on_syntax_test/11: Fail # Issue 19823
 deep_nesting1_negative_test: Crash # Issue 25557
 deep_nesting2_negative_test: Crash # Issue 25557
 
-regress_18713_test: Fail # Issue 26743
-
 call_function_apply_test: RuntimeError # Issue 23873
 mixin_supertype_subclass_test: CompileTimeError # Issue 23773
 mixin_supertype_subclass2_test: CompileTimeError # Issue 23773
@@ -224,8 +222,6 @@ metadata_test: CompileTimeError # Issue 5841
 infinity_test: RuntimeError # Issue 4984
 mixin_mixin2_test: RuntimeError # Issue 13109.
 mixin_mixin3_test: RuntimeError # Issue 13109.
-mixin_mixin7_test: RuntimeError # Issue 13109.
-mixin_regress_13688_test: RuntimeError # Issue 13109.
 modulo_test: RuntimeError # Issue 15246
 truncdiv_test: RuntimeError # Issue 15246
 invocation_mirror2_test: RuntimeError # Issue 6490 (wrong retval).