fix swapLocations - when !withLocation && withSource

Author: ichiriac

src/ast.js

@@ -360,15 +360,12 @@ AST.prototype.prepare = function(kind, docs, parser) {
       if (self.withSource) {
         src = parser.lexer._input.substring(start.offset, parser.prev[2]);
       }
-      if (self.withPositions) {
-        location = new Location(
-          src,
-          start,
-          new Position(parser.prev[0], parser.prev[1], parser.prev[2])
-        );
-      } else {
-        location = new Location(src, null, null);
-      }
+      // if with source, need location on swapLocations function
+      location = new Location(
+        src,
+        start,
+        new Position(parser.prev[0], parser.prev[1], parser.prev[2])
+      );
       // last argument is allways the location
       args.push(location);
     }