Mark extensionRPCs as optional

runtime/vm/service/service.md

-# Dart VM Service Protocol 3.2
+# Dart VM Service Protocol 3.3
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
-This document describes of _version 3.2_ of the Dart VM Service Protocol. This
+This document describes of _version 3.3_ 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,
 but this does not allow access to VM _events_ and is not documented
 here.
 
 The Service Protocol uses [JSON-RPC 2.0][].
 
@@ skipping 1901 matching lines @@
 
   // A list of all breakpoints for this isolate.
   Breakpoint[] breakpoints;
 
   // The error that is causing this isolate to exit, if applicable.
   Error error [optional];
 
   // The current pause on exception mode for this isolate.
   ExceptionPauseMode exceptionPauseMode;
 
-  // The list of service extension RPCs that are registered for this isolate.
-  string[] extensionRPCs;
+  // The list of service extension RPCs that are registered for this isolate,
+  // if any.
+  string[] extensionRPCs [optional];
 }
 ```
 
 An _Isolate_ object provides information about one isolate in the VM.
 
 ### Library
 
 ```
 class @Library extends @Object {
   // The name of this library.
@@ skipping 544 matching lines @@
 version | comments
 ------- | --------
 1.0 | initial revision
 2.0 | Describe protocol version 2.0.
 3.0 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.  Added Sentinel return to getIsolate.  Add AddedBreakpointWithScriptUri.  Removed Isolate.entry. The type of VM.pid was changed from string to int.  Added VMUpdate events.  Add offset and count parameters to getObject() and offset and count fields to Instance. Added ServiceExtensionAdded event.
 3.1 | Add the getSourceReport RPC.  The getObject RPC now accepts offset and count for string objects.  String objects now contain length, offset, and count properties.
 3.2 | Isolate objects now include the runnable bit and many debugger related RPCs will return an error if executed on an isolate before it is runnable.
 3.3 | Pause event now indicates if the isolate is paused at an await, yield, or yield* suspension point via the 'atAsyncSuspension' field. Resume command now supports the step parameter 'OverAsyncSuspension'. A Breakpoint added synthetically by an 'OverAsyncSuspension' resume command identifies itself as such via the 'isSyntheticAsyncContinuation' field.
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss