Add dart:developer.postEvent for posting events to the service protocol from Dart code

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,
@@ skipping 40 matching lines @@
         - [Class](#class)
         - [ClassList](#classlist)
         - [Code](#code)
         - [CodeKind](#codekind)
         - [Context](#context)
         - [ContextElement](#contextelement)
         - [Error](#error)
         - [ErrorKind](#errorkind)
         - [Event](#event)
         - [EventKind](#eventkind)
+  - [ExtensionData](#extensiondata)
         - [Field](#field)
         - [Flag](#flag)
         - [FlagList](#flaglist)
         - [Frame](#frame)
         - [Function](#function)
         - [Instance](#instance)
         - [Isolate](#isolate)
         - [Library](#library)
         - [LibraryDependency](#librarydependency)
         - [MapAssociation](#mapassociation)
@@ skipping 640 matching lines @@
 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
 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
 Stderr | WriteEvent
 
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   // This is provided for the WriteEvent event.
   string bytes [optional];
 
   // The argument passed to dart:developer.inspect.
   //
   // This is provided for the Inspect event.
   @Instance inspectee [optional];
 
   // The RPC name of the extension that was added.
   //
-  // This is provided for the ServiceExtensionAdded.
+  // This is provided for the ServiceExtensionAdded event.
   string extensionRPC [optional];
+
+  // The extension event kind.
+  //
+  // This is provided for the Extension event.
+  string extensionKind [optional];
+
+  // The extension event data.
+  //
+  // This is provided for the Extension event.
+  ExtensionData extensionData [optional];
 }
 ```
 
 An _Event_ is an asynchronous notification from the VM. It is delivered
 only when the client has subscribed to an event stream using the
 [streamListen](#streamListen) RPC.
 
 For more information, see [events](#events).
 
 ### EventKind
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   // A breakpoint has been removed.
   BreakpointRemoved,
 
   // A garbage collection event.
   GC,
 
   // Notification of bytes written, for example, to stdout/stderr.
   WriteEvent,
 
   // Notification from dart:developer.inspect.
-  Inspect
+  Inspect,
+
+  // Event from dart:developer.postEvent.
+  Extension
 }
 ```
 
 Adding new values to _EventKind_ is considered a backwards compatible
 change. Clients should ignore unrecognized events.
 
+### ExtensionData
+
+```
+class ExtensionData {
+}
+```
+
+An _ExtensionData_ is an arbitrary map that can have any contents.
+
 ### Field
 
 ```
 class @Field extends @Object {
   // The name of this field.
   string name;
 
   // The owner of this field, which can be either a Library or a
   // Class.
   @Object owner;
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 ## Revision History
 
 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.
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss