[VM,libs] Add workaround for issue 31305.

The tear-off of _NativeSocket.multiplex needs to have runtime type
`(Object) -> void` in order to be passed to the RawReceivePort
constructor.  Once issue #31305 is fixed, we should be able to fix
this by marking _NativeSocket.multiplex's argument as "covariant".
Until then, we have to type the argument as `Object` and then assign
it.

Change-Id: I1c9b7fb77dd3b0a71037459206f8de30ad77f73e
Reviewed-on: https://dart-review.googlesource.com/20822
Reviewed-by: Sigmund Cherem <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

runtime/bin/socket_patch.dart

@@ -811,7 +811,10 @@ class _NativeSocket extends _NativeSocketNativeWrapper with _ServiceObject {
   }
 
   // Multiplexes socket events to the socket handlers.
-  void multiplex(int events) {
+  void multiplex(Object eventsObj) {
+    // TODO(paulberry): when issue #31305 is fixed, we should be able to simply
+    // declare `events` as a `covariant int` parameter.
+    int events = eventsObj;
     for (int i = FIRST_EVENT; i <= LAST_EVENT; i++) {
       if (((events & (1 << i)) != 0)) {
         if ((i == CLOSED_EVENT || i == READ_EVENT) && isClosedRead) continue;