Dart语言方法覆盖

import 'dart:io'; 
void main() { 
   Child c = new Child(); 
   c.m1(12); 
} 
class Parent { 
   void m1(int a){ print("value of a ${a}");} 
} 
class Child extends Parent { 
   @override 
   void m1(String b) { 
      print("value of b ${b}");
   } 
}