be more explicit about when and how to register for listeners with isolates

sdk/lib/isolate/isolate.dart

Index: sdk/lib/isolate/isolate.dart
diff --git a/sdk/lib/isolate/isolate.dart b/sdk/lib/isolate/isolate.dart
index 8372eafbc169c2562f212ff442b810ec352a6a54..9b488c21bb243150f8e52c68dd02c3034e105f67 100644
--- a/sdk/lib/isolate/isolate.dart
+++ b/sdk/lib/isolate/isolate.dart
@@ -276,6 +276,10 @@ class Isolate {
    * If `response` cannot be sent to the isolate, then the request is ignored.
    * It is recommended to only use simple values that can be sent to all
    * isolates, like `null`, booleans, numbers or strings.
+   * 
+   * Since isolates run concurrently, it's possible for it to exit before the
+   * exit listener is established. To avoid this, start the isolate paused,
+   * add the listener, then resume it.
    */
   /* TODO(lrn): Can we do better? Can the system recognize this message and
    * send a reply if the receiving isolate is dead?
@@ -364,6 +368,10 @@ class Isolate {
    *
    * Listening using the same port more than once does nothing. It will only
    * get each error once.
+   * 
+   * Since isolates run concurrently, it's possible for it to exit before the

[nweiz, 2015/06/12 19:18:55]

"exit" -> "emit an error"

[Lasse Reichstein Nielsen, 2015/06/12 19:38:16]

"exit or emit an error"?

It can do either or both.

[nweiz, 2015/06/12 21:05:43]

That's true, but if it exits before the listener is established the listener
just won't fire—the same as if it exits afterwards. I think the unintuitive
behavior is particular to emitting errors.

+   * error listener is established. To avoid this, start the isolate paused,
+   * add the listener, then resume it.
    */
   external void addErrorListener(SendPort port);