2nd attempt at adding streamListen/streamCancel to the service protocol.

runtime/vm/service/service.md

 # Dart VM Service Protocol 1.0 (Draft 1)
 
 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.
 It is possible to make HTTP (non-WebSocket) requests,
 but this does not allow access to VM _events_ and is not documented
@@ skipping 853 matching lines @@
   // The number of variables in this context.
   int length;
 }
 ```
 
 ```
 class Context {
   // The number of variables in this context.
   int length;
 
+  // The enclosing context for this context.
+  Context parent [optional];
+
   // The variables in this context object.
   ContextElement[] variables;
 }
 ```
 
 ### ContextElement
 
 ```
 class ContextElement {
   @Instance|Sentinel value;
@@ skipping 987 matching lines @@
   // A list of isolates running in the VM.
   @Isolate[] isolates
 }
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 1.0 draft 1 | initial revision