Fix all strong-mode warnings.

lib/src/credentials.dart

Index: lib/src/credentials.dart
diff --git a/lib/src/credentials.dart b/lib/src/credentials.dart
index 49d2a6415975b1a1ddb0da2030bc5bc564d13ff1..14809e15dacab8960ec32040100fb50de7828074 100644
--- a/lib/src/credentials.dart
+++ b/lib/src/credentials.dart
@@ -132,7 +132,7 @@ class Credentials {
         parsed['accessToken'],
         refreshToken: parsed['refreshToken'],
         tokenEndpoint: tokenEndpoint,
-        scopes: scopes,
+        scopes: (scopes as List).map((scope) => scope as String),
         expiration: expiration);
   }
 
@@ -184,7 +184,7 @@ class Credentials {
           "endpoint.");
     }
 
-    var headers = {};
+    var headers = <String, String>{};
 
     var body = {
       "grant_type": "refresh_token",