[Firebase Auth] added access to User metadata

allows to recover the account creation date, and the last sign in date
of an user.

With them, you can determine if a user logs in for the first time.
(and the, create a welcome/ setup page, for example)

packages/firebase_auth/android/src/main/java/io/flutter/plugins/firebaseauth/FirebaseAuthPlugin.java

@@ -613,6 +613,8 @@ private ImmutableMap<String, Object> mapFromUser(FirebaseUser user) {
       }
       ImmutableMap<String, Object> userMap =
           userInfoToMap(user)
+              .put("creationTimestamp", user.getMetadata().getCreationTimestamp())
+              .put("lastSignInTimestamp", user.getMetadata().getLastSignInTimestamp())
               .put("isAnonymous", user.isAnonymous())
               .put("isEmailVerified", user.isEmailVerified())
               .put("providerData", providerDataBuilder.build())

packages/firebase_auth/ios/Classes/FirebaseAuthPlugin.m

@@ -276,6 +276,8 @@ - (NSMutableDictionary *)dictionaryFromUser:(FIRUser *)user {
     [providerData addObject:toDictionary(userInfo)];
   }
   NSMutableDictionary *userData = [toDictionary(user) mutableCopy];
+  userData[@"creationTimestamp"] = user.metadata.creationDate;
+  userData[@"lastSignInTimestamp"] = user.metadata.lastSignInDate;
   userData[@"isAnonymous"] = [NSNumber numberWithBool:user.isAnonymous];
   userData[@"isEmailVerified"] = [NSNumber numberWithBool:user.isEmailVerified];
   userData[@"providerData"] = providerData;

packages/firebase_auth/lib/firebase_auth.dart

@@ -8,6 +8,16 @@ import 'package:flutter/services.dart';
 import 'package:meta/meta.dart';
 
 /// Represents user data returned from an identity provider.
+
+class FirebaseUserMetadata {
+  final Map<dynamic, dynamic> _data;
+
+  FirebaseUserMetadata._(this._data);
+
+  int get creationTimestamp => _data['creationTimestamp'];
+  int get lastSignInTimestamp => _data['lastSignInTimestamp'];
+}
+
 class UserInfo {
   final Map<dynamic, dynamic> _data;
 
@@ -57,16 +67,20 @@ class UserUpdateInfo {
 /// Represents a user.
 class FirebaseUser extends UserInfo {
   final List<UserInfo> providerData;
+  final FirebaseUserMetadata _metadata;
 
   FirebaseUser._(Map<dynamic, dynamic> data)
       : providerData = data['providerData']
             .map<UserInfo>((dynamic item) => new UserInfo._(item))
             .toList(),
+        _metadata = new FirebaseUserMetadata._(data),
         super._(data);
 
   // Returns true if the user is anonymous; that is, the user account was
   // created with signInAnonymously() and has not been linked to another
   // account.
+  FirebaseUserMetadata get metadata => _metadata;
+
   bool get isAnonymous => _data['isAnonymous'];
 
   /// Returns true if the user's email is verified.