Only stop HTTP server if it was actually started.
Only stop HTTP server if it was actually started.

Review URL: https://codereview.chromium.org//14052008

git-svn-id: a0fc6a377c96b9059f80895148836233a16d98be@21939 260f80e4-7a28-3924-810f-c04153c831b5

kasperpeulen/http,dart-lang/http,dart-lang/http,dart-lang/http,dart-lang/http,dart-lang/http,dart-lang/http,donny-dont/http,dart-lang/http,dart-lang/http