anchor.diff:      if (!analysis.containsSource(source)) {
         CompilationUnit unit = engine.getCompilationUnit(source); 
         element = new SourceElement(source, unit);
  +      analysis.addSource(source, element);
         this.visitCompilationUnit(unit);
       } else {
         element = analysis.getSource(source);
  
positive.code:   }
    
    visitImportDirective(ImportDirective node) {
      engine.getSource(source, node);
    }
  }
negative.code:   List<FunctionElement> functions = <FunctionElement>[];
    List<VariableElement> top_variables = <VariableElement>[];
    
    SourceElement(Source this.source, CompilationUnit this.ast);
    
  }
  
  