Add empty mapping entries for the ranges of generated JavaScript that don't map to any Dart source …

lib/compiler/implementation/js/printer.dart

 // Copyright (c) 2012, 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.
 
 class Printer implements NodeVisitor {
   final bool shouldCompressOutput;
   leg.Compiler compiler;
   var positionElement;
   leg.CodeBuffer outBuffer;
   int indentLevel = 0;
@@ skipping 41 matching lines @@
     }
   }
 
   void recordSourcePosition(var position) {
     if (position != null) {
       outBuffer.setSourceLocation(positionElement, position);
     }
   }
 
   visit(Node node) {
+    if (node.sourcePosition != null) outBuffer.beginMappedRange();
     recordSourcePosition(node.sourcePosition);
     node.accept(this);
     recordSourcePosition(node.endSourcePosition);
+    if (node.sourcePosition != null) outBuffer.endMappedRange();
   }
 
   visitCommaSeparated(List<Node> nodes, int hasRequiredType,
                       [bool newInForInit, bool newAtStatementBegin]) {
     for (int i = 0; i < nodes.length; i++) {
       if (i != 0) {
         atStatementBegin = false;
         out(",");
         spaceOut();
       }
@@ skipping 743 matching lines @@
 }
 
 
 leg.CodeBuffer prettyPrint(Node node,
                            leg.Compiler compiler,
                            Dynamic positionElement) {
   Printer printer = new Printer(compiler, positionElement);
   printer.visit(node);
   return printer.outBuffer;
 }