getIsolate rpc now returns collected sentinel for expired isolate.

runtime/vm/service/service.md

 # Dart VM Service Protocol 3.0
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 3.0_ of the Dart VM Service Protocol. This
 protocol is used to communicate with a running Dart Virtual Machine.
 
 To use the Service Protocol, start the VM with the *--observe* flag.
 The VM will start a webserver which services protocol requests via WebSocket.
 It is possible to make HTTP (non-WebSocket) requests,
@@ skipping 460 matching lines @@
 ```
 
 The _getFlagList RPC returns a list of all command line flags in the
 VM along with their current values.
 
 See [FlagList](#flaglist).
 
 ### getIsolate
 
 ```
-Isolate getIsolate(string isolateId)
+Isolate|Sentinel getIsolate(string isolateId)
 ```
 
 The _getIsolate_ RPC is used to lookup an _Isolate_ object by its _id_.
 
+If _isolateId_ refers to an isolate which has exited, then the
+_Collected_ [Sentinel](#sentinel) is returned.
+
 See [Isolate](#isolate).
 
 ### getObject
 
 ```
-Object|Sentinel  getObject(string isolateId,
-                           string objectId)
+Object|Sentinel getObject(string isolateId,
+                          string objectId)
 ```
 
 The _getObject_ RPC is used to lookup an _object_ from some isolate by
 its _id_.
 
 If _objectId_ is a temporary id which has expired, then then _Expired_
 [Sentinel](#sentinel) is returned.
 
 If _objectId_ refers to a heap object which has been collected by the VM's
 garbage collector, then the _Collected_ [Sentinel](#sentinel) is
@@ skipping 1603 matching lines @@
   @Isolate[] isolates;
 }
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 1.0 draft 1 | initial revision
 1.1 | Describe protocol version 2.0.
-1.2 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.
+1.2 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.  Added Sentinel return to getIsolate.
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss