Remove sideeffectful position mutation.

packages/babel-generator/src/buffer.js

@@ -333,7 +333,7 @@ export default class Buffer {
     }
 
     // If there the line is ending, adding a new mapping marker is redundant
-    if (this.opts.sourceMaps && str[0] !== "\n") this.map.mark(this._sourcePosition);
+    if (this.opts.sourceMaps && str[0] !== "\n") this.map.mark(this.position, this._sourcePosition);
 
     //
     this.position.push(str);

packages/babel-generator/src/index.js

@@ -31,7 +31,7 @@ class Generator extends Printer {
     this._inForStatementInitCounter = 0;
 
     this.whitespace = new Whitespace(tokens);
-    this.map        = new SourceMap(position, opts, code);
+    this.map        = new SourceMap(opts, code);
   }
 
   format: {

packages/babel-generator/src/source-map.js

@@ -5,8 +5,7 @@ import sourceMap from "source-map";
  */
 
 export default class SourceMap {
-  constructor(position, opts, code) {
-    this.position = position;
+  constructor(opts, code) {
     this.opts     = opts;
     this.last     = {generated: {}, original: {}};
 
@@ -46,12 +45,10 @@ export default class SourceMap {
    * values to insert a mapping to nothing.
    */
 
-  mark(sourcePos: Object) {
+  mark(position, sourcePos: Object) {
     let map = this.map;
     if (!map) return; // no source map
 
-    let position = this.position;
-
     // Adding an empty mapping at the start of a generated line just clutters the map.
     if (this._lastGenLine !== position.line && sourcePos.line === null) return;