[dart:io] flush() stdin before close()ing

sdk/lib/io/io_sink.dart

Index: sdk/lib/io/io_sink.dart
diff --git a/sdk/lib/io/io_sink.dart b/sdk/lib/io/io_sink.dart
index b1c05a02542b9a412e1440f49c83df226210fe42..fe3eb9f57735d91aaeff1a49cff3f4601746764b 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -123,6 +123,10 @@ abstract class IOSink implements StreamSink<List<int>>, StringSink {
 
   /**
    * Close the target consumer.
+   *
+   * NOTE: Writes to the [IOSink] may be buffered, and may not be flushed by the
+   * a call to `close()`. To flush all buffered writes, call `flush()` before
+   * calling `close()`.
    */
   Future close();