Document that IOSink.add may buffer data.

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 4dab40fd6e031ad458c66156aad0b2a2ab79ce2d..351a30bc10301f0e3ff52bc1d71f0b02505abc92 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -26,7 +26,9 @@ abstract class IOSink implements StreamSink<List<int>>, StringSink {
   Encoding encoding;
 
   /**
-   * Writes the bytes uninterpreted to the consumer.
+   * Writes the bytes uninterpreted to the consumer. While the call is
+   * synchronous, the data may be buffered until the underlying resource is
+   * ready. The data should not be modified after a call to [add].
    */
   void add(List<int> data);