public Object recordingMethod() {
    if (methodCalled) {
      throw new IllegalStateException("Method called more than once.");
    }
    methodCalled = true;
    return methodReturnValue;
  }
Records the invocation in {@link #methodCalled} and returns the value in
{@link #FIXTURE_RETURN_VALUE}.
otto/src/test/java/com/squareup/otto/EventProducerTest.java