Remove the systemFetch export from platform_node (#1963)

BUG

Author: Nikhil Thorat

tfjs-core/src/platforms/platform_node.ts

@@ -24,7 +24,19 @@ export const getNodeFetch = {
   importFetch: () => require('node-fetch')
 };
 
-export let systemFetch: (url: string, init?: RequestInit) => Promise<Response>;
+type FetchFn = (url: string, init?: RequestInit) => Promise<Response>;
+let systemFetch: FetchFn;
+// These getters and setters are for testing so we don't export a mutable
+// variable.
+export function resetSystemFetch() {
+  systemFetch = null;
+}
+export function setSystemFetch(fetchFn: FetchFn) {
+  systemFetch = fetchFn;
+}
+export function getSystemFetch(): FetchFn {
+  return systemFetch;
+}
 
 export class PlatformNode implements Platform {
   private textEncoder: TextEncoder;

tfjs-core/src/platforms/platform_node_test.ts

@@ -42,11 +42,10 @@ describeWithFlags('PlatformNode', NODE_ENVS, () => {
 
     const platform = new PlatformNode();
 
-    const savedFetch = platform_node.systemFetch;
+    const savedFetch = platform_node.getSystemFetch();
 
     // Null out the system fetch so we force it to require node-fetch.
-    // @ts-ignore
-    platform_node.systemFetch = null;
+    platform_node.resetSystemFetch();
 
     const testFetch = {fetch: (url: string, init: RequestInit) => {}};
 
@@ -63,8 +62,7 @@ describeWithFlags('PlatformNode', NODE_ENVS, () => {
     expect(platform_node.getNodeFetch.importFetch).toHaveBeenCalled();
     expect(testFetch.fetch).toHaveBeenCalledWith('test/url', {method: 'GET'});
 
-    // @ts-ignore
-    platform_node.systemFetch = savedFetch;
+    platform_node.setSystemFetch(savedFetch);
     ENV.global.fetch = globalFetch;
   });