dart - Modifying strings with source_span package in dartlang? -

i want transform parts of source files own transformer in dart. have dart file input , want edit file. know can contents of file with:

  var content = transform.primaryinput.readasstring(encoding: utf8); 

from inside transformers. know how find offset in source want apply changes.

is there kind of "cursor"-style editing package strings in dart? want kind of (pseudo-code!):

string sourcestring = transform.primaryinput.readasstring(encoding: utf8); //sourcefile comes source_span package, wrapped cursor implementation sourcefile source = new sourcefile(sourcestring); var cursor = new cursor(sourcefile);  //.jumpto(int linenumber, [int columnnumber]) cursor.jumpto(30); //deletes next 2 chars cursor.delete(2); //adds new text after cursor cursor.write("hello world!"); //accept sourcespans deletion cursor.delete(sourcespanobject) 

or there better way change contents of source files in transfomers in dart?