Revert "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 ddc4618e87c12c47dd9c8198dc7426ff519e10ff..42c1db9843c22ec0041a4231469a7118a70beefe 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -940,7 +940,6 @@ class _HttpOutgoing implements StreamConsumer<List<int>> {
             "Headers size exceeded the of '$_OUTGOING_BUFFER_SIZE'"
             " bytes"));
       }
-      return null;
     }
     if (headersWritten) return null;
     headersWritten = true;
@@ -1914,8 +1913,6 @@ class _HttpClient implements HttpClient {
         return it.current;
       }
     }
-
-    return null;
   }
 
   void _removeCredentials(_Credentials cr) {