Provide stdout and stderr output in the Observatory debugger.

runtime/vm/service/service.md

 # Dart VM Service Protocol 1.0 (Draft 1)
 
 > Please post feedback to the [observatory-discuss group][discuss-list]
 
 This document describes _draft 1_ of _version 1.0_ 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.
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 subscribed) error code is returned.
 
 The _streamId_ parameter may have the following published values:
 
 streamId | event types provided
 -------- | -----------
 Isolate | IsolateStart, IsolateExit, IsolateUpdate
 Debug | PauseStart, PauseExit, PauseBreakpoint, PauseInterrupted, PauseException, Resume, BreakpointAdded, BreakpointResolved, BreakpointRemoved, Inspect
 GC | GC
 
+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
+
 It is considered a _backwards compatible_ change to add a new type of event to an existing stream.
 Clients should be written to handle this gracefully, perhaps by warning and ignoring.
 
 See [Success](#success).
 
 ## Public Types
 
 The following is a list of all public types produced by the Service Protocol.
 
 We define a small set of primitive types, based on JSON equivalents.
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   //   PauseException
   //
   // 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.
+  //
+  // This is provided for the WriteEvent event.
+  string bytes [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 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.
-  GC
+  GC,
+
+  // Notification of bytes written, for example, to stdout/stderr.
+  WriteEvent
 }
 ```
 
 Adding new values to _EventKind_ is considered a backwards compatible
 change. Clients should ignore unrecognized events.
 
 ### Field
 
 ```
 class @Field extends @Object {
@@ skipping 912 matching lines @@
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 1.0 draft 1 | initial revision
 
 
 [discuss-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/observatory-discuss