Provide list of service protocol extensions in isolate and emit an event when one is registered

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,
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 subscribed, the client will begin receiving events from the stream.
 
 If the client is not subscribed to the stream, the _103_ (Stream already
 subscribed) error code is returned.
 
 The _streamId_ parameter may have the following published values:
 
 streamId | event types provided
 -------- | -----------
 VM | VMUpdate
-Isolate | IsolateStart, IsolateRunnable, IsolateExit, IsolateUpdate
+Isolate | IsolateStart, IsolateRunnable, IsolateExit, IsolateUpdate, ServiceExtensionAdded
 Debug | PauseStart, PauseExit, PauseBreakpoint, PauseInterrupted, PauseException, Resume, BreakpointAdded, BreakpointResolved, BreakpointRemoved, Inspect
 GC | GC
 
 Additionally, some embedders provide the _Stdout_ and _Stderr_
 streams.  These streams allow the client to subscribe to writes to
 stdout and stderr.
 
 streamId | event types provided
 -------- | -----------
 Stdout | WriteEvent
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   // An array of bytes, encoded as a base64 string.
   //
   // This is provided for the WriteEvent event.
   string bytes [optional];
 
   // The argument passed to dart:developer.inspect.
   //
   // This is provided for the Inspect event.
   @Instance inspectee [optional];
+
+  // The RPC name of the extension that was added.
+  //
+  // This is provided for the ServiceExtensionAdded.
+  string extensionRPC [optional];
 }
 ```
 
 An _Event_ is an asynchronous notification from the VM. It is delivered
 only when the client has subscribed to an event stream using the
 [streamListen](#streamListen) RPC.
 
 For more information, see [events](#events).
 
 ### EventKind
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   IsolateRunnable,
 
   // Notification that an isolate has exited.
   IsolateExit,
 
   // Notification that isolate identifying information has changed.
   // Currently used to notify of changes to the isolate debugging name
   // via setName.
   IsolateUpdate,
 
+  // Notification that an extension RPC was registered on an isolate.
+  ServiceExtensionAdded,
+
   // An isolate has paused at start, before executing code.
   PauseStart,
 
   // An isolate has paused at exit, before terminating.
   PauseExit,
 
   // An isolate has paused at a breakpoint or due to stepping.
   PauseBreakpoint,
 
   // An isolate has paused due to interruption via pause.
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   @Library[] libraries;
 
   // 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;
 }
 ```
 
 An _Isolate_ object provides information about one isolate in the VM.
 
 ### Library
 
 ```
 class @Library extends @Object {
   // The name of this library.
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   @Isolate[] isolates;
 }
 ```
 
 ## Revision History
 
 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.
+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.
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss