Implement onClosed on a socket output stream

runtime/bin/socket_stream_impl.dart

Index: runtime/bin/socket_stream_impl.dart
diff --git a/runtime/bin/socket_stream_impl.dart b/runtime/bin/socket_stream_impl.dart
index ac750b9c091bf8be33c60f7458d78103de011f97..45c7e4d0e763aab00932557f986642d494efa6a6 100644
--- a/runtime/bin/socket_stream_impl.dart
+++ b/runtime/bin/socket_stream_impl.dart
@@ -109,6 +109,7 @@ class _SocketOutputStream
   }
 
   void close() {
+    if (_closing && _closed) return;
     if (!_pendingWrites.isEmpty()) {
       // Mark the socket for close when all data is written.
       _closing = true;
@@ -117,6 +118,10 @@ class _SocketOutputStream
       // Close the socket for writing.
       _socket._closeWrite();
       _closed = true;
+      // Invoke the callback asynchronously.
+      new Timer(0, (t) {
+        if (_onClosed != null) _onClosed();
+      });
     }
   }
 
@@ -134,6 +139,10 @@ class _SocketOutputStream
     }
   }
 
+  void set onClosed(void callback()) {
+    _onClosed = callback;
+  }
+
   bool _write(List<int> buffer, int offset, int len, bool copyBuffer) {
     if (_closing || _closed) throw new StreamException("Stream closed");
     int bytesWritten = 0;
@@ -176,6 +185,9 @@ class _SocketOutputStream
     if (_closing) {
       _socket._closeWrite();
       _closed = true;
+      if (_onClosed != null) {
+        _onClosed();
+      }
     } else {
       if (_onNoPendingWrites != null) _onNoPendingWrites();
     }
@@ -199,6 +211,7 @@ class _SocketOutputStream
   Socket _socket;
   _BufferList _pendingWrites;
   Function _onNoPendingWrites;
+  Function _onClosed;
   bool _closing = false;
   bool _closed = false;
 }