fix: adds check for Reflect. Reflect.getOwnMetadataKeys (#305)

Author: ChristianKienle

Diff for: src/reflect.ts

@@ -1,7 +1,10 @@
 import Vue, { VueConstructor } from 'vue'
 import { VueClass } from './declarations'
 
-export const reflectionIsSupported = typeof Reflect !== 'undefined' && Reflect.defineMetadata
+// The rational behind the verbose Reflect-feature check below is the fact that there are polyfills
+// which add an implementation for Reflect.defineMetadata but not for Reflect.getOwnMetadataKeys.
+// Without this check consumers will encounter hard to track down runtime errors.
+export const reflectionIsSupported = typeof Reflect !== 'undefined' && Reflect.defineMetadata && Reflect.getOwnMetadataKeys
 
 export function copyReflectionMetadata (
   to: VueConstructor,