Add operators test to source_mapping_test.

tests/compiler/dart2js/sourcemaps/sourcemap_helper.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library sourcemap.helper;
 
 import 'dart:async';
 import 'package:compiler/src/apiimpl.dart' as api;
 import 'package:compiler/src/dart2jslib.dart' show NullSink;
 import 'package:compiler/src/elements/elements.dart';
@@ skipping 235 matching lines @@
          codePoints.add(new CodePoint(kind, jsCode, sourceLocation,
              dartCodeFromSourceLocation(sourceLocation)));
       }
     }
 
     Map<int, List<SourceLocation>> locationMap = nodeMap[node];
     if (locationMap == null) {
       addLocation(null, nodeToString(node));
     } else {
       locationMap.forEach((int targetOffset, List<SourceLocation> locations) {
-        String jsCode = positionToString(targetOffset);
+        String jsCode = nodeToString(node);
         for (SourceLocation location in locations) {
           addLocation(location, jsCode);
         }
       });
     }
   }
 
   void apply(js.Node node) {
     node.accept(this);
   }
@@ skipping 59 matching lines @@
       this.jsCode,
       this.sourceLocation,
       this.dartCode,
       {this.isMissing: false});
 
   String toString() {
     return 'CodePoint[kind=$kind,js=$jsCode,dart=$dartCode,'
                      'location=$sourceLocation]';
   }
 }