Fix some service protocol isolate lifecycle races

runtime/vm/service/service.md

 # Dart VM Service Protocol 3.3
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes of _version 3.3_ 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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 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, ServiceExtensionAdded
-Debug | PauseStart, PauseExit, PauseBreakpoint, PauseInterrupted, PauseException, Resume, BreakpointAdded, BreakpointResolved, BreakpointRemoved, Inspect
+Debug | PauseStart, PauseExit, PauseBreakpoint, PauseInterrupted, PauseException, Resume, BreakpointAdded, BreakpointResolved, BreakpointRemoved, Inspect, None
 GC | GC
 Extension | Extension
 
 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 isolate has paused due to interruption via pause.
   PauseInterrupted,
 
   // An isolate has paused due to an exception.
   PauseException,
 
   // An isolate has started or resumed execution.
   Resume,
 
+  // Indicates an isolate is not yet runnable. Only appears in an Isolate's
+  // pauseEvent. Never sent over a stream.
+  None,
+
   // A breakpoint has been added for an isolate.
   BreakpointAdded,
 
   // An unresolved breakpoint has been resolved for an isolate.
   BreakpointResolved,
 
   // A breakpoint has been removed.
   BreakpointRemoved,
 
   // A garbage collection event.
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 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. Added ServiceExtensionAdded event.
 3.1 | Add the getSourceReport RPC.  The getObject RPC now accepts offset and count for string objects.  String objects now contain length, offset, and count properties.
 3.2 | Isolate objects now include the runnable bit and many debugger related RPCs will return an error if executed on an isolate before it is runnable.
 3.3 | Pause event now indicates if the isolate is paused at an await, yield, or yield* suspension point via the 'atAsyncSuspension' field. Resume command now supports the step parameter 'OverAsyncSuspension'. A Breakpoint added synthetically by an 'OverAsyncSuspension' resume command identifies itself as such via the 'isSyntheticAsyncContinuation' field.
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss