Fix #1444 - broken mutator for text_charat (#1451)

* Fix #1444 - broken mutator for text_charat

There might no longer be an input named AT on a text_charat block
which the mutator had been depending on. There is now a boolean on
the block for isAt, though, so use that directly instead.

The

* Address comments

blocks/text.js

@@ -806,8 +806,7 @@ Blockly.Constants.Text.TEXT_CHARAT_MUTATOR_MIXIN = {
    */
   mutationToDom: function() {
     var container = document.createElement('mutation');
-    var isAt = this.getInput('AT').type == Blockly.INPUT_VALUE;
-    container.setAttribute('at', isAt);
+    container.setAttribute('at', !!this.isAt_);
     return container;
   },
   /**