Don't fire change event on fields that haven't been named yet.

core/field.js

@@ -156,10 +156,6 @@ Blockly.Field.prototype.init = function() {
       Blockly.bindEvent_(this.fieldGroup_, 'mouseup', this, this.onMouseUp_);
   // Force a render.
   this.updateTextNode_();
-  if (Blockly.Events.isEnabled()) {
-    Blockly.Events.fire(new Blockly.Events.Change(
-        this.sourceBlock_, 'field', this.name, '', this.getValue()));
-  }
 };
 
 /**