public void recordingMethod(Object arg) {
    if (methodCalled) {
      throw new IllegalStateException("Method called more than once.");
    }
    methodCalled = true;
    methodArgument = arg;
  }
Records the provided object in {@link #methodArgument} and sets
{@link #methodCalled}.

@param arg  argument to record.
otto/src/test/java/com/squareup/otto/EventHandlerTest.java