code: public final void addClassesToModify(String... classes) {
          if (classes != null) {
              for (String clazz : classes) {
                  if (!shouldDefer(PACKAGES_TO_BE_DEFERRED, clazz)) {
                      modify.add(clazz);
                  }
              }
          }
      }
docs: Add classes that will be loaded by the mock classloader, i.e. these
  classes will be byte-code manipulated to allow for testing. Any classes
  contained in the {@link #PACKAGES_TO_BE_DEFERRED} will be ignored. How ever
  classes added here have precedence over additionally deferred (ignored)
  packages (those ignored by the user using @PrepareForTest).
  
  @param classes The fully qualified name of the classes that will be appended
  to the list of classes that will be byte-code modified to
  enable testability.