Update Stream.listen doc and use ?? in some places.

sdk/lib/async/stream.dart

Index: sdk/lib/async/stream.dart
diff --git a/sdk/lib/async/stream.dart b/sdk/lib/async/stream.dart
index c2b6c68984ad5dde9097cc28cd5247060e89d2a4..510cccb93d0b9f1044d696bd35b464479e91821d 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -325,6 +325,10 @@ abstract class Stream<T> {
    * two arguments it is called with the stack trace (which could be `null` if
    * the stream itself received an error without stack trace).
    * Otherwise it is called with just the error object.
+   * If [onError] is omitted, any errors on the stream are considered unhandled,
+   * and will be passed to the current [Zone]'s error handler.
+   * By default unhandled async errors are treated
+   * as if they were uncaught top-level errors.
    *
    * If this stream closes, the [onDone] handler is called.
    *