Fix typo in mixin test.

Change-Id: I1c6300dfa250ef0b9ea84e9bd5fd233eb5827464
Reviewed-on: https://dart-review.googlesource.com/74960
Commit-Queue: Lasse R.H. Nielsen <[email protected]>
Reviewed-by: Lasse R.H. Nielsen <[email protected]>

Author: munificent

tests/language_2/mixin_declaration/mixin_declaration_syntax_test.dart

@@ -382,5 +382,5 @@ void main() {
     }
   }
 
-  Expect equals(CeOwithM().toString(), CwithM().toString());
+  Expect.equals(CeOwithM().toString(), CwithM().toString());
 }