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

runtime/vm/service/service.md

Index: runtime/vm/service/service.md
diff --git a/runtime/vm/service/service.md b/runtime/vm/service/service.md
index ae3bf2fdfd9d8160f91d8582969718eeff393575..9867afad0c6c3678efea4797649e4b7e3ab389b6 100644
--- a/runtime/vm/service/service.md
+++ b/runtime/vm/service/service.md
@@ -620,7 +620,7 @@ 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
 
@@ -1026,6 +1026,9 @@ 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,
 
@@ -1590,9 +1593,6 @@ class Isolate extends Response {
   // 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;
 
@@ -1603,17 +1603,26 @@ class Isolate extends Response {
   // 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];
 }
 ```