Use define_method instead of class_eval & string.

This will make it easier to introduce a check at invocation time which compares
the number of arguments passed into the stubbed method and the "arity" of the
original method implementation.

lib/mocha/class_method.rb

@@ -54,11 +54,10 @@ def hide_original_method
     end
 
     def define_new_method
-      stubbee.__metaclass__.class_eval(%{
-        def #{method}(*args, &block)
-          mocha.method_missing(:#{method}, *args, &block)
-        end
-      }, __FILE__, __LINE__)
+      class_method = self
+      stubbee.__metaclass__.send(:define_method, method) do |*args, &block|
+        mocha.method_missing(class_method.method, *args, &block)
+      end
       if @original_visibility
         Module.instance_method(@original_visibility).bind(stubbee.__metaclass__).call(method)
       end