anchor.diff:    void _preVisit(AssignedVariables<Node, Var> assignedVariables) {
       _body._preVisit(assignedVariables);
     }
  -
  -  void _visit(Harness h) {
  -    h._flow.tryCatchStatement_catchBegin(_exception, _stackTrace);
  -    _body._visit(h);
  -    h._flow.tryCatchStatement_catchEnd();
  -  }
   }
   
   class _CheckAssigned extends Statement {
  
positive.code: 
    @override
    void _visit(Harness h) {
      h._flow.handleContinue(h._currentContinueTarget!);
    }
  }
  
  
negative.code: 
    @override
    void _visit(Harness h) {
      if (initializer != null) {
        h._typeAnalyzer.dispatchStatement(initializer!);
      } else {
        h._typeAnalyzer.handleNoInitializer();
      }
      h._flow.for_conditionBegin(this);
      if (condition != null) {
        h._typeAnalyzer.analyzeExpression(condition!);
      } else {
        h._typeAnalyzer.handleNoCondition();
      }
      h._flow.for_bodyBegin(forCollection ? null : this, condition);
      h._typeAnalyzer._visitLoopBody(this, body);
      h._flow.for_updaterBegin();
      if (updater != null) {
        h._typeAnalyzer.analyzeExpression(updater!);
      } else {
        h._typeAnalyzer.handleNoStatement();
      }
      h._flow.for_end();
    }
  }
  