Fix issue 476

[ccantill]

SetterObserver doesn't work if a property is already defined.

[jdanyow]

closing because the root cause is the way babel transpiles the decorated props- see #406 for more info.

[doktordirk]

@jdanyow @EisenbergEffect
a) i think the issue is more loganfsmyth/babel-plugin-transform-decorators-legacy#44 & tc39/proposal-class-public-fields#36
b) since you tell everyone to use that implementation of decorators, you can.t just get away with saying, "not our problem" and not trying to get it fixed
c) while i don.t think this fix is right, the basic idea hold s true. whatever the reason for not being able to use the getter/setter, it should fall back to dirty-checking, which from looking at the code it doesn.t

@ccantill you can fix it locally by adding @observable to your decorated proberties or use a fixed decorators implementation npm i github:doktordirk/babel-plugin-transform-decorators-legacy#release --save-dev

[ccantill]

@jdanyow I agree that the way babel transpiles the decorated props wasn't optimal and that that fixes it, but I still think aurelia should fall back to dirty checking if it can't reconfigure the property rather than just failing silently. I'm sure there could be cases where a non-reconfigurable property needs to be observed without the possibility of changing the code for the property (third party libraries for example).

@doktordirk thanks! I was indeed using @observable as a temporary workaround.

[jdanyow]

@doktordirk / @ccantill We can't add dirty-checking to fix a transpilation issue because there are many members of the community that are trying to avoid dirty-checking at all costs. They would consider this fix to be a bug.