Use service library from Observatory

[johnmccutchan]

Service library in Observatory can handle all communication over the service (to content_shell, standalone, and Chrome VM) and provides model objects for responses. It should allow you to drop a bunch of code.

[cbracken]

f3dfba44dd038742857c1b9c01adbaf2080f6838 killed off devtools.dart and moved us over to package:vm_service_client, which technically resolves this issue. Unfortunately it also increased the set of transitive deps from args, logging, and path to 36 packages.

Until we've got that issue (#109) sorted, I'd prefer to keep this issue open.

[cbracken]

Now using the vm_service_client package.