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

lib/compiler/implementation/code_buffer.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 CodeBuffer implements StringBuffer {
   StringBuffer buffer;
   List<SourceLocation> sourceLocations;
   int lastBufferOffset = 0;
+  int mappedRangeCounter = 0;
 
   CodeBuffer()
       : buffer = new StringBuffer(),
         sourceLocations = new List<SourceLocation>();
 
   int get length => buffer.length;
 
   bool isEmpty() {
     return buffer.isEmpty();
   }
 
   /**
    * Converts [object] to a string and adds it to the buffer. If [object] is a
    * [CodeBuffer], adds its source locations to [sourceLocations].
    */
   CodeBuffer add(var object) {
     if (object is CodeBuffer) {
       return addBuffer(object);
     }
+    if (mappedRangeCounter == 0) setSourceLocation(null, null);
     buffer.add(object.toString());
     return this;
   }
 
   CodeBuffer addBuffer(CodeBuffer other) {
     if (other.sourceLocations.length > 0) {
       SourceLocation firstMapping = other.sourceLocations[0];
       int offsetDelta =
           buffer.length + firstMapping.offsetDelta - lastBufferOffset;
       sourceLocations.add(new SourceLocation(firstMapping.element,
@@ skipping 22 matching lines @@
     buffer.clear();
     sourceLocations.clear();
     lastBufferOffset = 0;
     return this;
   }
 
   String toString() {
     return buffer.toString();
   }
 
+  void beginMappedRange() {
+    ++mappedRangeCounter;
+  }
+
+  void endMappedRange() {
+    assert(mappedRangeCounter > 0);
+    --mappedRangeCounter;
+  }
+
   void setSourceLocation(Element element, Token token) {
     int offsetDelta = buffer.length - lastBufferOffset;
     sourceLocations.add(new SourceLocation(element, token, offsetDelta));
     lastBufferOffset = buffer.length;
   }
 
   void forEachSourceLocation(void f(Element element, Token token, int offset)) {
     int offset = 0;
     sourceLocations.forEach((sourceLocation) {
       offset += sourceLocation.offsetDelta;
       f(sourceLocation.element, sourceLocation.token, offset);
     });
   }
 }
 
 class SourceLocation {
   final Element element;
   final Token token;
   final int offsetDelta;
   SourceLocation(this.element, this.token, this.offsetDelta);
 }