Don't crash when HSV_VALUE == 1

256 isn't a valid value (valid values range from 0-255, inclusive).  So the correct constant to multiply by is 255.

core/blockly.js

@@ -101,7 +101,7 @@ Blockly.SPRITE = {
  */
 Blockly.makeColour = function(hue) {
   return goog.color.hsvToHex(hue, Blockly.HSV_SATURATION,
-      Blockly.HSV_VALUE * 256);
+      Blockly.HSV_VALUE * 255);
 };
 
 /**