anchor.diff:      var libUri = lib.fileUri;
       var importUri = lib.importUri;
       var source = uriToSource[libUri];
  -    if (source == null) return null;
  +    if (source == null) {
  +      // Sources that only contain external declarations have nothing to add to
  +      // the sum.
  +      continue;
  +    }
       dartSize += source.source.length;
       for (var part in lib.parts) {
         var partUri = part.partUri;
  
positive.code:   return CompilerResult(0);
  }
  
  // Compute code size to embed in the generated JavaScript
  // for this module.  Return `null` to indicate when size could not be properly
  // computed for this module.
  int _computeDartSize(Component component) {
    var dartSize = 0;
    var uriToSource = component.uriToSource;
  
negative.code:       }
        var fileUri = libUri.resolve(partUri);
        var partSource = uriToSource[fileUri];
        if (partSource == null) {
          // Sources that only contain external declarations have nothing to add
          // to the sum.
          continue;
        }
        dartSize += partSource.source.length;
      }
    }
  