monaco: don't call change handler for read only files.
monaco: don't call change handler for read only files.

Microsoft/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,Microsoft/pxt,playi/pxt,playi/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt,switchinnovations/pxt,playi/pxt