Add a message for analysis server to indicate a successful connection.

Add a message for analysis server to indicate a successful connection.

This complements the "server already started" error introduced in
r35046.  It's now easy for the client to detect whether a connection
to the server was successful--it simply connects and waits for the
initial message from the server.  The message returned on a successful
connection is:

{"event":"server.connected"}

R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35108

[Brian Wilkerson, 2014-04-16 02:53:44]

LGTM

[Brian Wilkerson, 2014-04-16 02:57:33]

Although, as I was about to add documentation for this I wondered whether this
might want to be a notification rather than a response.

[Paul Berry, 2014-04-16 03:25:08]

On 2014/04/16 02:57:33, Brian Wilkerson wrote:
> Although, as I was about to add documentation for this I wondered whether this
> might want to be a notification rather than a response.

You've got a good point.  As a notification it would probably look something
like:

{"event":"connected","params":{}}

which seems quite a bit cleaner than:

{"id":"","error":null,"result":{"connected":true}}

I'll plan on changing it tomorrow morning unless someone weighs in with a
dissenting opinion.

[Brian Wilkerson, 2014-04-16 14:04:09]

> {"event":"connected","params":{}}

I added the notification to the spec, but in keeping with the domain model I set
the name as "server.connected".

[Paul Berry, 2014-04-16 15:11:41]

Committed patchset #1 manually as r35108 (presubmit successful).