So siehts ganz gut aus !

node_modules/core-js/library/modules/es7.reflect.define-metadata.js

@@ -0,0 +1,8 @@
+var metadata = require('./_metadata');
+var anObject = require('./_an-object');
+var toMetaKey = metadata.key;
+var ordinaryDefineOwnMetadata = metadata.set;
+
+metadata.exp({ defineMetadata: function defineMetadata(metadataKey, metadataValue, target, targetKey) {
+  ordinaryDefineOwnMetadata(metadataKey, metadataValue, anObject(target), toMetaKey(targetKey));
+} });