Add an IsolateRunnable event to the service protocol. Improve service docs.

runtime/vm/service/service.md

 # Dart VM Service Protocol 2.0
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 2.0_ 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 602 matching lines @@
 The _streamListen_ RPC subscribes to a stream in the VM. Once
 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
 -------- | -----------
-Isolate | IsolateStart, IsolateExit, IsolateUpdate
+Isolate | IsolateStart, IsolateRunnable, IsolateExit, IsolateUpdate
 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
@@ skipping 385 matching lines @@
 
 For more information, see [events](#events).
 
 ### EventKind
 
 ```
 enum EventKind {
   // Notification that a new isolate has started.
   IsolateStart,
 
+  // Notification that an isolate is ready to run.
+  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,
 
   // An isolate has paused at start, before executing code.
   PauseStart,
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   string number;
 
   // A name identifying this isolate. Not guaranteed to be unique.
   string name;
 
   // The time that the VM started in milliseconds since the epoch.
   //
   // Suitable to pass to DateTime.fromMillisecondsSinceEpoch.
   int startTime;
 
-  // The entry function for this isolate.
-  @Function entry [optional];
-
   // The number of live ports for this isolate.
   int livePorts;
 
   // Will this isolate pause when exiting?
   bool pauseOnExit;
 
   // The last pause event delivered to the isolate. If the isolate is
   // running, this will be a resume event.
   Event pauseEvent;
 
-  // The error that is causing this isolate to exit, if applicable.
-  Error error [optional];
+  // The entry function for this isolate.
+  //
+  // Guaranteed to be initialized when the IsolateRunnable event fires.
+  @Function entry [optional];
 
   // The root library for this isolate.
-  @Library rootLib;
+  //
+  // Guaranteed to be initialized when the IsolateRunnable event fires.
+  @Library rootLib [optional];
 
   // A list of all libraries for this isolate.
+  //
+  // Guaranteed to be initialized when the IsolateRunnable event fires.
   @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];
 }
 ```
 
 An _Isolate_ object provides information about one isolate in the VM.
 
 ### Library
 
 ```
 class @Library extends @Object {
   // The name of this library.
@@ skipping 372 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