Fix #1397 by being more careful with mutator (#1410)
Fix #1397 by being more careful with mutator (#1410)

* Fix #1397 by being more careful with mutator

The text_charAt mutator was replacing more of the title string than
necessary, which would lead to losing some of the relevant text in
the label. Updated the mutator to only add/remove a new input on the
end instead of replacing the default dummy input.

* Code cleanup

rachel-fenichel/blockly,google/blockly,nickmain/blockly,twodee/blockly,joshlory/blockly,nickmain/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,jstnhuang/blockly,groklearning/blockly,trodi/blockly,joshlory/blockly,twodee/blockly,picklesrus/blockly,jstnhuang/blockly,google/blockly,google/blockly,twodee/blockly,trodi/blockly,groklearning/blockly,trodi/blockly,trinketapp/blockly,trinketapp/blockly,rachel-fenichel/blockly,picklesrus/blockly,mark-friedman/blockly,trinketapp/blockly,google/blockly,mark-friedman/blockly,mark-friedman/blockly,mark-friedman/blockly,twodee/blockly,nickmain/blockly,groklearning/blockly,mark-friedman/blockly,google/blockly,nickmain/blockly,nickmain/blockly,nickmain/blockly,google/blockly,trinketapp/blockly,picklesrus/blockly,mark-friedman/blockly,joshlory/blockly,google/blockly,trodi/blockly,rachel-fenichel/blockly,rachel-fenichel/blockly,twodee/blockly,twodee/blockly,jstnhuang/blockly,jstnhuang/blockly,groklearning/blockly