use textSpanEnd instead of handrolled version

Author: vladima

src/services/services.ts

@@ -1836,11 +1836,10 @@ module ts {
  ? sourceFile.text.substr(0, textChangeRange.span.start)
  : "";
 
- let textChangeRangeEnd = textChangeRange.span.start + textChangeRange.span.length;
  // grab the fragment from the end of the span till the end of the original text
- let suffix = textChangeRangeEnd !== sourceFile.text.length
-  ? sourceFile.text.substr(textChangeRangeEnd)
-  : "";
+ let suffix = textSpanEnd(textChangeRange.span) !== sourceFile.text.length
+ ? sourceFile.text.substr(textSpanEnd(textChangeRange.span))
+ : "";
 
  if (textChangeRange.newLength === 0) {
  // edit was a deletion - just combine prefix and suffix