do not apply polyfills if not necessary

src-node/polyfills.ts

@@ -5,18 +5,29 @@ import { EventEmitter } from 'events';
 import { Blob } from 'buffer';
 import fetch from 'node-fetch';
 
-global.Blob = Blob;
-
-global.fetch = fetch;
+if (!('Blob' in global)) {
+  global.Blob = Blob;
+}
+if (!('fetch' in global)) {
+  global.fetch = fetch;
+}
 
 // mimic window events
 const emitter = new EventEmitter();
-global.addEventListener = emitter.addListener.bind(emitter);
-global.removeEventListener = emitter.removeListener.bind(emitter);
-global.dispatchEvent = (event) => emitter.emit(event.type, event);
-global.CustomEvent = class {
-  constructor(type, payload) {
-    this.type = type;
-    Object.assign(this, payload);
-  }
-};
+if (
+  !('addEventListener' in global) &&
+  !('removeEventListener' in global) &&
+  !('dispatchEvent' in global)
+) {
+  global.addEventListener = emitter.addListener.bind(emitter);
+  global.removeEventListener = emitter.removeListener.bind(emitter);
+  global.dispatchEvent = emitter.emit.bind(emitter);
+}
+if (!('CustomEvent' in global)) {
+  global.CustomEvent = class {
+    constructor(type, payload) {
+      this.type = type;
+      Object.assign(this, payload);
+    }
+  };
+}