Fix warnings and hints in the SDK

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index dd03aeb7218d5d54a50f19eb7534c2386965360f..a7b990bd47193490b43be0b990b263ef5f8f248a 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -940,6 +940,7 @@ class _HttpOutgoing implements StreamConsumer<List<int>> {
             "Headers size exceeded the of '$_OUTGOING_BUFFER_SIZE'"
             " bytes"));
       }
+      return null;
     }
     if (headersWritten) return null;
     headersWritten = true;
@@ -1909,6 +1910,8 @@ class _HttpClient implements HttpClient {
         return it.current;
       }
     }
+
+    return null;
   }
 
   void _removeCredentials(_Credentials cr) {