Make VM service id handling JSON-RPC 2 compliant.

runtime/vm/service/service.md

 # Dart VM Service Protocol 1.0 (Draft 1)
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes _draft 1_ of _version 1.0_ 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.
@@ skipping 79 matching lines @@
 
 ```
 {
   "jsonrpc": "2.0",
   "method": "getVersion",
   "params": {},
   "id": "1"
 }
 ```
 
-Currently the _id_ property must be a string. The Service Protocol
+The _id_ property must be a string, number, or `null`. The Service Protocol
 optionally accepts requests without the _jsonprc_ property.
 
 An RPC response is a JSON object (http://json.org/). The response always specifies an
 _id_ property to pair it with the corresponding request. If the RPC
 was successful, the _result_ property provides the result.
 
 Here is an example response for our [getVersion](#getversion) request above:
 
 ```
 {
@@ skipping 1876 matching lines @@
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 1.0 draft 1 | initial revision
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss