Refactor isolate interrupts to use OOB messages instead of interrupt bits.

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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   //   PauseBreakpoint
   Breakpoint[] pauseBreakpoints [optional];
 
   // The top stack frame associated with this event, if applicable.
   //
   // This is provided for the event kinds:
   //   PauseBreakpoint
   //   PauseInterrupted
   //   PauseException
   //
+  // For PauseInterrupted events, there will be no top frame if the
+  // isolate is idle (waiting in the message loop).
+  //
   // For the Resume event, the top frame is provided at
   // all times except for the initial resume event that is delivered
   // when an isolate begins execution.
   Frame topFrame [optional];
 
   // The exception associated with this event, if this is a
   // PauseException event.
   @Instance exception [optional];
 
   // An array of bytes, encoded as a base64 string.
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 ## Revision History
 
 version | comments
 ------- | --------
 1.0 draft 1 | initial revision
 1.1 | Describe protocol version 2.0.
 1.2 | Describe protocol version 3.0.  Added UnresolvedSourceLocation.
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss