Replace knownEvents modules with static strings.

ui/core/proxy.ts

@@ -95,7 +95,7 @@ export class ProxyObject extends bindable.Bindable implements definition.ProxyOb
             proxyMetadata.onSetNativeValue({
                 object: this,
                 property: property,
-                eventName: observable.knownEvents.propertyChange,
+                eventName: observable.Observable.propertyChangeEvent,
                 newValue: newValue,
                 oldValue: oldValue
             });