fix more mixin tests to work in strong mode

tests/language_strong/mixin_type_parameters_simple_test.dart

Index: tests/language_strong/mixin_type_parameters_simple_test.dart
diff --git a/tests/language_strong/mixin_type_parameters_simple_test.dart b/tests/language_strong/mixin_type_parameters_simple_test.dart
index b8ab415bf408480efa474fe792445fc4fe7ec5ee..8721735f73c90b8415a4e1e6221bf8286b3c7ae3 100644
--- a/tests/language_strong/mixin_type_parameters_simple_test.dart
+++ b/tests/language_strong/mixin_type_parameters_simple_test.dart
@@ -23,7 +23,7 @@ main() {
   // so we make sure to handle these specifically in the compiler.
   Expect.equals("int", a.m1().toString());
   Expect.equals("int", a.m2().toString());
-  a = new A<String>();
-  Expect.equals("String", a.m1().toString());
-  Expect.equals("String", a.m2().toString());
+  var a2 = new A<String>();
+  Expect.equals("String", a2.m1().toString());
+  Expect.equals("String", a2.m2().toString());
 }