PASS: agent_server.HttpServer exists
start() return a Future that completes

PASS: agent_server.HttpServer.start() return a Future that completes
Renamed HttpServer to WebServer.

To not conflict wit dart:io HttpServer
WebServer.httpServer starts out as null

PASS: agent_server.WebServer.httpServer starts out as null
stared server open on port 8080

PASS: agent_server.WebServer.start() httpServer is now a stared server
open on port 8080
getWebServer() returns a instace of WebServer

PASS: agent_server.Agent.getWebServer() returns a instace of WebServer
Fixsed start() to use new getWebServer.