Add js-compat flag.

[back2dos]

To accompany https://github.com/haxetink/tink_core/issues/161,

• make it so that bind and value are available on all ObservableObject implementors at runtime (and for StateObject there should be a setter for value).
• make sure Promised is usable
• the runtime version of auto will be interesting ... perhaps there should simply be an asyncAuto for async computations