anchor.diff:        }
         var fileUri = libUri.resolve(partUri);
         var partSource = uriToSource[fileUri];
  -      if (partSource == null) return null;
  +      if (partSource == null) {
  +        // Sources that only contain external declarations have nothing to add
  +        // to the sum.
  +        continue;
  +      }
         dartSize += partSource.source.length;
       }
     }
  
positive.code:     var libUri = lib.fileUri;
      var importUri = lib.importUri;
      var source = uriToSource[libUri];
      if (source == null) return null;
      dartSize += source.source.length;
      for (var part in lib.parts) {
        var partUri = part.partUri;
  
negative.code:   return CompilerResult(0);
  }
  
  /// Compute code size to embed in the generated JavaScript for this module.
  int _computeDartSize(Component component) {
    var dartSize = 0;
    var uriToSource = component.uriToSource;
  