Rework the format of the FlagList response.

runtime/vm/service/service.md

 # Dart VM Service Protocol 0.0
 
 This document describes _version 0.0_ 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,
 but this does not allow access to VM _events_ and is not documented
@@ skipping 1054 matching lines @@
 ### Flag
 
 ```
 class Flag {
   // The name of the flag.
   string name;
 
   // A description of the flag.
   string comment;
 
+  // Has this flag been modified from its default setting?
+  bool modified;
+
   // The value of this flag as a string.
   //
   // If this property is absent, then the value of the flag was NULL.
   string valueAsString [optional];
 }
 ```
 
 A _Flag_ represents a single VM command line flag.
 
 ### FlagList
@@ skipping 739 matching lines @@
   // A list of isolates running in the VM.
   @Isolate[] isolates
 }
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 0.0 | draft