Warn when adding something to a closed sink and improve documentation

tests/standalone/io/file_test.dart

Index: tests/standalone/io/file_test.dart
diff --git a/tests/standalone/io/file_test.dart b/tests/standalone/io/file_test.dart
index c3d9e6b3b23c4abfa944d881a16d857da458292a..0176033ba57b0b25e13c22198770cd7adb5e9d79 100644
--- a/tests/standalone/io/file_test.dart
+++ b/tests/standalone/io/file_test.dart
@@ -988,7 +988,9 @@ class FileTest {
     file.createSync();
     var output = file.openWrite();
     output.close();
-    output.add(buffer); // Ignored.
+    Expect.throws(() {
+      output.add(buffer);
+    });
     output.done.then((_) {
       file.deleteSync();
       asyncTestDone("testCloseExceptionStream");