Move div by zero testing into its own test.

tests/language/div_with_power_of_two_test.dart

Index: tests/language/div_with_power_of_two_test.dart
===================================================================
--- tests/language/div_with_power_of_two_test.dart	(revision 27742)
+++ tests/language/div_with_power_of_two_test.dart	(working copy)
@@ -98,8 +98,5 @@
         Expect.equals(res, f(arg));
       }
     }
-    Expect.throws(() => divBy0(4),
-                  (e) => e is IntegerDivisionByZeroException
-                         || e is UnsupportedError);
   }
 }