Fix all strong-mode errors and warnings.

lib/src/json_document_transformer.dart

Index: lib/src/json_document_transformer.dart
diff --git a/lib/src/json_document_transformer.dart b/lib/src/json_document_transformer.dart
index 19b2e08f75e0f4818125ed8b67a0386501a409e9..c62c597cb549c8c4b48746faf5116b94edd76b12 100644
--- a/lib/src/json_document_transformer.dart
+++ b/lib/src/json_document_transformer.dart
@@ -22,7 +22,7 @@ final jsonDocument = new JsonDocumentTransformer();
 /// [FormatException]. If an unencodable object is added to the sink, it
 /// synchronously throws a [JsonUnsupportedObjectError].
 class JsonDocumentTransformer
-    implements StreamChannelTransformer<String, Object> {
+    implements StreamChannelTransformer<Object, String> {
   /// The underlying codec that implements the encoding and decoding logic.
   final JsonCodec _codec;