Procedures with arguments break

[NeilFraser]

Paste the following XML into the playground:

<xml xmlns="https://developers.google.com/blockly/xml">
  <variables>
    <variable type="" id="~{V?,c5*l[5ZIwfmf*(,">x</variable>
    <variable type="" id="ID`:z38s-(tm,)[di1o]">y</variable>
  </variables>
  <block type="procedures_defnoreturn" id="A]`k~7~hfP1`@1;]-Sjc" x="187" y="-12">
    <mutation>
      <arg name="x" varid="~{V?,c5*l[5ZIwfmf*(,"/>
      <arg name="y" varid="ID`:z38s-(tm,)[di1o]"/>
    </mutation>
    <field name="NAME">proc</field>
  </block>
</xml>

On the resulting procedure block, click the mutator button. On the console is this error:

procedures.js:173 Uncaught TypeError: node.appendChild is not a function
    at Blockly.BlockSvg.decompose (procedures.js:173)
    at Blockly.Mutator.setVisible (mutator.js:270)
    at Blockly.Mutator.Blockly.Icon.iconClick_ (icon.js:131)
    at Blockly.Mutator.iconClick_ (mutator.js:116)
    at SVGGElement.wrapFunc (blockly.js:481)

Initially I assumed it was my fault since I've been mucking with XML recently. But
it looks like it belongs to @BeksOmega.

[NeilFraser]

Confirming fixed. Thanks!