fix more mixin tests to work in strong mode

tests/language_strong/mixin_type_parameter4_test.dart

Index: tests/language_strong/mixin_type_parameter4_test.dart
diff --git a/tests/language_strong/mixin_type_parameter4_test.dart b/tests/language_strong/mixin_type_parameter4_test.dart
index 575058a5b58c3e89439892aa449ca6d9ea0baf21..d309b7f326422e60e8161a02a770f7c892ac4949 100644
--- a/tests/language_strong/mixin_type_parameter4_test.dart
+++ b/tests/language_strong/mixin_type_parameter4_test.dart
@@ -17,8 +17,8 @@ class A1<T> extends B1 with M<T> {}
 class A2<T> = B2 with M<T>;
 
 main() {
-  var ab = new A1<int>();
-  Expect.isTrue(ab is R<bool, int>);
-  ab = new A2<int>();
-  Expect.isTrue(ab is R<bool, int>);
+  var a1 = new A1<int>();
+  Expect.isTrue(a1 is R<bool, int>);
+  var a2 = new A2<int>();
+  Expect.isTrue(a2 is R<bool, int>);
 }