Warn when adding something to a closed sink and improve documentation

sdk/lib/core/sink.dart

Index: sdk/lib/core/sink.dart
diff --git a/sdk/lib/core/sink.dart b/sdk/lib/core/sink.dart
index 0d8f2eff254a9528e114c2d0d083265b617b0f52..c79214bbf5dac7060b715d298111480a6b101522 100644
--- a/sdk/lib/core/sink.dart
+++ b/sdk/lib/core/sink.dart
@@ -14,18 +14,18 @@ part of dart.core;
  */
 abstract class Sink<T> {
   /**
-   * Put the data into the sink.
+   * Adds [data] to the sink.
    *
    * Must not be called after a call to [close].
    */
   void add(T data);
 
   /**
-   * Tell the sink that no further data will be added.
-   *
-   * Calling this method more than once is allowed, but does nothing.
+   * Closes the sink.
    *
    * The [add] method must not be called after this method.
+   *
+   * Calling this method more than once is allowed, but does nothing.
    */
   void close();
 }