Update or delete language tests using obsolete interfaces or default factory

tests/language/method_override2_test.dart

Index: tests/language/method_override2_test.dart
===================================================================
--- tests/language/method_override2_test.dart	(revision 15648)
+++ tests/language/method_override2_test.dart	(working copy)
@@ -4,13 +4,13 @@
 
 // Checks that an overriding method has compatible parameters.
 
-interface I {
+abstract class I {
   m({a, b});
 }
 
-interface J extends I { }
+abstract class J extends I { }
 
-interface K extends J {
+abstract class K extends J {
   m({c, d});  /// 00: compile-time error
 }