Don't emit repeated null SourceLocations

sdk/lib/_internal/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.
 
 part of dart2js;
 
 class CodeBuffer implements StringBuffer {
 
   StringBuffer buffer = new StringBuffer();
   List<CodeBufferMarker> markers = new List<CodeBufferMarker>();
@@ skipping 86 matching lines @@
   void beginMappedRange() {
     ++mappedRangeCounter;
   }
 
   void endMappedRange() {
     assert(mappedRangeCounter > 0);
     --mappedRangeCounter;
   }
 
   void setSourceLocation(var sourcePosition) {
+    if (sourcePosition == null) {
+      if (markers.length > 0 && markers.last.sourcePosition == null) return;
+    }
     int offsetDelta = buffer.length - lastBufferOffset;
     markers.add(new CodeBufferMarker(offsetDelta, sourcePosition));
     lastBufferOffset = buffer.length;
   }
 
   void forEachSourceLocation(void f(int targetOffset, var sourcePosition)) {
     int targetOffset = 0;
     markers.forEach((marker) {
       targetOffset += marker.offsetDelta;
       f(targetOffset, marker.sourcePosition);
     });
   }
 }
 
 class CodeBufferMarker {
   final int offsetDelta;
   final sourcePosition;
 
   CodeBufferMarker(this.offsetDelta, this.sourcePosition);
 }