Add flush to HTTP output streams

runtime/bin/http_impl.dart

Index: runtime/bin/http_impl.dart
diff --git a/runtime/bin/http_impl.dart b/runtime/bin/http_impl.dart
index b92b4b81517aacb527c0be555827263a6aa27b46..da64e52f5467d561e84aa70400904e60e638a497 100644
--- a/runtime/bin/http_impl.dart
+++ b/runtime/bin/http_impl.dart
@@ -1034,6 +1034,10 @@ class _HttpResponse extends _HttpRequestResponseBase implements HttpResponse {
     return _writeList(buffer, offset, len);
   }
 
+  void _streamFlush() {
+    _httpConnection._flush();
+  }
+
   void _streamClose() {
     _responseEnd();
   }
@@ -1197,10 +1201,6 @@ class _HttpInputStream extends _BaseDataInputStream implements InputStream {
     return result;
   }
 
-  void flush() {
-    // Nothing to do on a HTTP output stream.
-  }
-
   void _close() {
     // TODO(sgjesse): Handle this.
   }
@@ -1224,6 +1224,10 @@ class _HttpOutputStream extends _BaseOutputStream implements OutputStream {
     return _requestOrResponse._streamWriteFrom(buffer, offset, len);
   }
 
+  void flush() {
+    _requestOrResponse._streamFlush();
+  }
+
   void close() {
     _requestOrResponse._streamClose();
   }
@@ -1279,6 +1283,10 @@ class _HttpConnectionBase {
     }
   }
 
+  bool _flush() {
+    _socket.outputStream.flush();
+  }
+
   bool _close() {
     _closing = true;
     _socket.outputStream.close();
@@ -1647,6 +1655,10 @@ class _HttpClientRequest
     return _writeList(buffer, offset, len);
   }
 
+  void _streamFlush() {
+    _httpConnection._flush();
+  }
+
   void _streamClose() {
     _ensureHeadersSent();
     _state = DONE;