Don't use null safety (yet) (#3112)

It raises errors when rolling into the sdk.
diff --git a/lib/src/authentication/client.dart b/lib/src/authentication/client.dart
index 8360e8a..db7e9d8 100644
--- a/lib/src/authentication/client.dart
+++ b/lib/src/authentication/client.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// ignore_for_file: import_of_legacy_library_into_null_safe
+// @dart=2.11
 
 import 'dart:io';
 
@@ -69,7 +69,7 @@
       // the hostedUrl? Or at least we might need to log request.url to give
       // user additional insights on what's happening.
 
-      String? serverMessage;
+      String serverMessage;
 
       try {
         final wwwAuthenticateHeaderValue =
@@ -108,7 +108,7 @@
       if (serverMessage?.isNotEmpty == true) {
         // Only allow printable ASCII, map anything else to whitespace, take
         // at-most 1024 characters.
-        final truncatedMessage = String.fromCharCodes(serverMessage!.runes
+        final truncatedMessage = String.fromCharCodes(serverMessage.runes
             .map((r) => 32 >= r && r <= 127 ? r : 32)
             .take(1024));
 
diff --git a/lib/src/authentication/credential.dart b/lib/src/authentication/credential.dart
index db3b308..d840f6b 100644
--- a/lib/src/authentication/credential.dart
+++ b/lib/src/authentication/credential.dart
@@ -2,7 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// ignore_for_file: import_of_legacy_library_into_null_safe
+// @dart=2.11
+
+import 'package:meta/meta.dart';
 
 import '../exceptions.dart';
 import '../source/hosted.dart';
@@ -23,9 +25,9 @@
 class Credential {
   /// Internal constructor that's only used by [fromJson].
   Credential._internal({
-    required this.url,
-    required this.token,
-    required this.unknownFields,
+    @required this.url,
+    @required this.token,
+    @required this.unknownFields,
   });
 
   /// Create a new [Credential].
@@ -57,7 +59,7 @@
   final Uri url;
 
   /// Authentication token value
-  final String? token;
+  final String token;
 
   /// Unknown fields found in pub-tokens.json. The fields might be created by the
   /// future version of pub tool. We don't want to override them when using the
diff --git a/lib/src/authentication/token_store.dart b/lib/src/authentication/token_store.dart
index 6f68fe6..24f70fc 100644
--- a/lib/src/authentication/token_store.dart
+++ b/lib/src/authentication/token_store.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// ignore_for_file: import_of_legacy_library_into_null_safe
+// @dart=2.11
 
 import 'dart:convert';
 
@@ -132,10 +132,10 @@
     return found;
   }
 
-  /// Returns [Credential] for authenticating given [hostedUrl] or null if no
+  /// Returns [Credential] for authenticating given [hostedUrl] or `null` if no
   /// matching credential is found.
-  Credential? findCredential(Uri hostedUrl) {
-    Credential? matchedCredential;
+  Credential findCredential(Uri hostedUrl) {
+    Credential matchedCredential;
     for (final credential in credentials) {
       if (credential.url == hostedUrl && credential.isValid()) {
         if (matchedCredential == null) {
diff --git a/lib/src/command/token.dart b/lib/src/command/token.dart
index bbe35a6..dafa88f 100644
--- a/lib/src/command/token.dart
+++ b/lib/src/command/token.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// ignore_for_file: import_of_legacy_library_into_null_safe
+// @dart=2.11
 
 import '../command.dart';
 import 'token_add.dart';
diff --git a/lib/src/command/token_add.dart b/lib/src/command/token_add.dart
index aa2a2f5..24840f2 100644
--- a/lib/src/command/token_add.dart
+++ b/lib/src/command/token_add.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// ignore_for_file: import_of_legacy_library_into_null_safe
+// @dart=2.11
 
 import 'dart:async';
 
diff --git a/lib/src/command/token_list.dart b/lib/src/command/token_list.dart
index 56d1cb4..828f004 100644
--- a/lib/src/command/token_list.dart
+++ b/lib/src/command/token_list.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// ignore_for_file: import_of_legacy_library_into_null_safe
+// @dart=2.11
 
 import '../command.dart';
 import '../log.dart' as log;
diff --git a/lib/src/command/token_remove.dart b/lib/src/command/token_remove.dart
index df038c2..5db8d51 100644
--- a/lib/src/command/token_remove.dart
+++ b/lib/src/command/token_remove.dart
@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// ignore_for_file: import_of_legacy_library_into_null_safe
+// @dart=2.11
 
 import '../command.dart';
 import '../exceptions.dart';