change back from const to let

Responding to PR comments, but it has to be let here.

Author: alan412

src/blocks/mrc_mechanism.ts

@@ -103,7 +103,7 @@ const MECHANISM_FUNCTION = {
      */
     updateBlock_: function(this: MechanismBlock): void {
       // Add input sockets for the arguments.
-      for (const i = 0; i < this.mrcArgs.length; i++) {
+      for (let i = 0; i < this.mrcArgs.length; i++) {
         const input = this.appendValueInput('ARG' + i)
             .setAlign(Blockly.inputs.Align.RIGHT)
             .appendField(this.mrcArgs[i].name);
@@ -129,7 +129,7 @@ export const pythonFromBlock = function (
   let code = 'self.mechanisms["' + mechanismBlock.getFieldValue('NAME') + '"] = '
     + mechanismBlock.getFieldValue('TYPE') + '('
 
-  for (const i = 0; i < mechanismBlock.mrcArgs.length; i++) {
+  for (let i = 0; i < mechanismBlock.mrcArgs.length; i++) {
       const fieldName = 'ARG' + i;
       if(i != 0){
         code += ', '