add system test, remove async on terminate

dev/src/index.ts

@@ -1012,8 +1012,8 @@ export class Firestore {
    *
    * @return A Promise that resolves when the client is terminated.
    */
-  async terminate(): Promise<void> {
-    await this._clientPool.terminate();
+  terminate(): Promise<void> {
+    return this._clientPool.terminate();
   }
 
   /**

dev/system-test/firestore.ts

@@ -127,6 +127,24 @@ describe('Firestore class', () => {
         expect(docs[1].data()).to.deep.equal({f: 'a'});
       });
   });
+
+  it('cannot make calls after the client has been terminated', () => {
+    const ref1 = randomCol.doc('doc1');
+    ref1.onSnapshot(snapshot => {
+      return Promise.reject('onSnapshot() should be called');
+    });
+    return firestore
+      .terminate()
+      .then(() => {
+        return ref1.set({foo: 100});
+      })
+      .then(() => {
+        Promise.reject('set() should have failed');
+      })
+      .catch(err => {
+        expect(err.message).to.equal('The client has already been terminated');
+      });
+  });
 });
 
 describe('CollectionReference class', () => {