Fix small style issues in the dart example, following Effective Dart:

- use final instead of obvious types for local variables
- add missing `void` return type
- make the comments doccomments (///)
diff --git a/examples/list_people.dart b/examples/list_people.dart
index dd5bb41..8a96d82 100644
--- a/examples/list_people.dart
+++ b/examples/list_people.dart
@@ -3,16 +3,16 @@
 import 'dart_tutorial/addressbook.pb.dart';
 import 'dart_tutorial/addressbook.pbenum.dart';
 
-// Iterates though all people in the AddressBook and prints info about them.
+/// Iterates though all people in the AddressBook and prints info about them.
 void printAddressBook(AddressBook addressBook) {
-  for (Person person in addressBook.people) {
+  for (var person in addressBook.people) {
     print('Person ID: ${person.id}');
     print('  Name: ${person.name}');
     if (person.hasEmail()) {
       print('  E-mail address:${person.email}');
     }
 
-    for (Person_PhoneNumber phoneNumber in person.phones) {
+    for (var phoneNumber in person.phones) {
       switch (phoneNumber.type) {
         case Person_PhoneType.MOBILE:
           print('   Mobile phone #: ');
@@ -32,16 +32,16 @@
   }
 }
 
-// Reads the entire address book from a file and prints all
-// the information inside.
-main(List<String> arguments) {
+/// Reads the entire address book from a file and prints all
+/// the information inside.
+void main(List<String> arguments) {
   if (arguments.length != 1) {
     print('Usage: list_person ADDRESS_BOOK_FILE');
     exit(-1);
   }
 
   // Read the existing address book.
-  File file = new File(arguments.first);
-  AddressBook addressBook = new AddressBook.fromBuffer(file.readAsBytesSync());
+  final file = new File(arguments.first);
+  final addressBook = new AddressBook.fromBuffer(file.readAsBytesSync());
   printAddressBook(addressBook);
 }