Remove deprecated Strings class.

pkg/oauth2/lib/src/utils.dart

Index: pkg/oauth2/lib/src/utils.dart
diff --git a/pkg/oauth2/lib/src/utils.dart b/pkg/oauth2/lib/src/utils.dart
index 83fe2b7012170130613d4bcba7555bf5d90e53ea..f69ac8b194436c61db0439ad6b40d4cccf0cba79 100644
--- a/pkg/oauth2/lib/src/utils.dart
+++ b/pkg/oauth2/lib/src/utils.dart
@@ -39,10 +39,10 @@ String mapToQuery(Map<String, String> map) {
     value = (value == null || value.isEmpty) ? null : encodeUriComponent(value);
     pairs.add([key, value]);
   });
-  return Strings.join(pairs.map((pair) {
+  return pairs.map((pair) {
     if (pair[1] == null) return pair[0];
     return "${pair[0]}=${pair[1]}";
-  }), "&");
+  }).join("&");
 }
 
 /// Add all key/value pairs from [source] to [destination], overwriting any