Deferred loading seems to work reasonably if the lazy imports are in the main library. However, if they're in an imported library, then part.js is not generated, and trying to load the library fails at runtime. You can see this in that

$dartj2s lazy.dart
Dart file lazy.dart compiled to JavaScript: out.js

$ d8 out.js
Before loading: null
Lib2:
  Two

but

$ dart2js lazyIndirectly.dart
Dart file lazyIndirectly.dart compiled to JavaScript: out.js
$ d8 out.js
Before loading: Closure
part.js:2: ReferenceError: $ is not defined
var old$ = $;
           ^
ReferenceError: $ is not defined
    at part.js:2:12
    at _load_closure0.call$0 (out.js:2257:5)
    at _DefaultZone._ZoneBase._runInZone$2 (out.js:2775:16)
    at _ZoneTimer._run$0 (out.js:2840:8)
    at BoundClosure$0.$$.BoundClosure$0.call$0 (out.js:25:36)
    at TimerImpl_internalCallback.call$0 (out.js:1392:21)
    at _IsolateContext.eval$1 (out.js:632:21)
    at _IsolateEvent.process$0 (out.js:757:18)
    at _EventLoop.runIteration$0 (out.js:713:12)
    at _EventLoop._runHelper$0 (out.js:720:19)

Uncaught Error: Error executing file
out.js:2215: Error executing file
      throw exception;
            ^
Error executing file
    at wrapException (out.js:1565:13)
    at _DefaultZone_handleUncaughtError_closure.call$0 (out.js:2824:13)
    at Closure$_asyncRunCallback._asyncRunCallback as call$0
    at TimerImpl_internalCallback.call$0 (out.js:1392:21)
    at _IsolateContext.eval$1 (out.js:632:21)
    at _IsolateEvent.process$0 (out.js:757:18)
    at _EventLoop.runIteration$0 (out.js:713:12)
    at _EventLoop._runHelper$0 (out.js:720:19)
    at _EventLoop.run$0 (out.js:726:12)
    at startRootIsolate (out.js:419:29)

Attachments:
lazy.dart (245 Bytes)
lazy2.dart (46 Bytes)
lazyIndirectly.dart (73 Bytes)