Target V8's experimental strong mode

[vsmenon]

Running Chrome with --js-flags="--strong-mode" and replacing all uses of 'use strict' with 'use strong', we get:

Chrome 47.0.2523 (Mac OS X 10.10.5) ERROR
  Uncaught SyntaxError: In strong mode, '==' and '!=' are deprecated, use '===' and '!==' instead
  at /Users/vsm/git/ddc-clean/lib/runtime/dart_library.js:60

[jmesserly]

well that one's easy to fix at least ;-)

Aside: ideally, they'd allow == null and != null which is probably what we're doing. It's a nice way to express "null or undefined"