How to match password and confirm password in Flutter?

Just simply declare a variable first then assign it's value to the value which we are using.

Then compare it down as shown below:

GlobalKey<FormState> _formKey = GlobalKey<FormState>();
      var confirmPass;
    TextFormField(
                                validator: (String value) {
                                  confirmPass = value;
                                  if (value.isEmpty) {
                                    return "Please Enter New Password";
                                  } else if (value.length < 8) {
                                    return "Password must be atleast 8 characters long";
                                  } else {
                                    return null;
                                  }
                                },
                                decoration: InputDecoration(
                                  hintText: "Enter New Password",
                                  hintStyle: TextStyle(color: Colors.grey),
                                  border: new OutlineInputBorder(
                                    borderRadius: const BorderRadius.all(
                                      const Radius.circular(40.0),
                                    ),
                                  ),
                                ),
                              ),
                            ),
                            SizedBox(height: 20),
                            Container(
                              child: TextFormField(
                                validator: (String value) {
                                  if (value.isEmpty) {
                                    return "Please Re-Enter New Password";
                                  } else if (value.length < 8) {
                                    return "Password must be atleast 8 characters long";
                                  } else if (value != confirmPass) {
                                    return "Password must be same as above";
                                  } else {
                                    return null;
                                  }
                                },
                                decoration: InputDecoration(
                                  hintText: "Re-Enter New Password",
                                  hintStyle: TextStyle(color: Colors.grey),
                                  border: new OutlineInputBorder(
                                    borderRadius: const BorderRadius.all(
                                      const Radius.circular(40.0),
                                    ),
                                  ),
                                ),
                              ),
                            ),

Use TextFormField widget which consists of a builtin validator

  // Form
  final GlobalKey<FormState> _form = GlobalKey<FormState>();
  final TextEditingController _pass = TextEditingController();
  final TextEditingController _confirmPass = TextEditingController();

  Form(
        key: _form,
        child: ListView(
              children: <Widget>[
                     TextFormField(
                           controller: _pass,
                           validator: (val){
                              if(val.isEmpty)
                                   return 'Empty';
                              return null;
                              }
                     ),
                      TextFormField(
                           controller: _confirmPass,
                           validator: (val){
                              if(val.isEmpty)
                                   return 'Empty';
                              if(val != _pass.text)
                                   return 'Not Match'
                              return null;
                              }
                     ),
                       ]
              )
    )


    // To validate call
    _form.currentState.validate()