class="parameter-name">start</span> = <span class="default-value">0</span></span> <span class="parameter" id="normalize-param-end"><span class="type-annotation"><a href="dart-core/int-class.html">int</a></span> <span class="parameter-name">end</span></span> ])
            <span class="returntype parameter">&#8594; <a href="dart-core/String-class.html">String</a></span>
          </span>
                  </dt>
        <dd>
          Validates and normalizes the base64 encoded data in <code>source</code>. <a