Support dart 1.13 (re: SecureSocket)

[srawlins]

http_server needs to be updated here and there to support Dart 1.13:

• test/utils.dart needs to not call SecureSocket.initialize.
• Other problems in tests, I think.

Dart has provided a migration guide. Um... it might be out of date though. SecureSocket.initialize doesn't exist anymore.

[kevmoo]

Great call on the migration guide, @srawlins – opened a bug on the site – dart-archive/www.dartlang.org#1520

[sgjesse]

Fixed in 3ed8c55.