anchor.diff:          return export;
         }
       }
  +    if (!_enclosingUnit.context.exists(source)) {
  +      return null;
  +    }
       _mismatch("Could not find export element for '$source'", node);
       return null; // Never reached
     }
  
positive.code:    */
    ImportElement _findImport(
        ImportDirective node, List<ImportElement> imports, Source source) {
      if (source == null || !_enclosingUnit.context.exists(source)) {
        return null;
      }
      SimpleIdentifier prefix = node.prefix;
  
negative.code:         // So, either the combinators are the same, or we build new elements.
          return element;
        }
      }
      if (!_enclosingUnit.context.exists(source)) {
        return null;
      }
      if (foundSource) {
        if (prefix == null) {
  