Add test to verify does not overwrite existing implementation

test/reflect-other.ts

@@ -0,0 +1,15 @@
+import * as fs from "fs";
+import { assert } from "chai";
+
+describe("Reflect", () => {
+    it("does not overwrite existing implementation", () => {
+        const defineMetadata = Reflect.defineMetadata;
+
+        const reflectPath = require.resolve("../Reflect.js");
+        const reflectContent = fs.readFileSync(reflectPath, "utf8");
+        const reflectFunction = Function(reflectContent);
+        reflectFunction();
+
+        assert.strictEqual(Reflect.defineMetadata, defineMetadata);
+    });
+});