remove unused import and added a test for core construct

packages/@aws-cdk/aws-eks/test/test.cluster.ts

@@ -9,7 +9,6 @@ import * as lambda from '@aws-cdk/aws-lambda';
 import * as cdk from '@aws-cdk/core';
 import * as cdk8s from 'cdk8s';
 import * as constructs from 'constructs';
-import { Construct } from 'constructs';
 import { Test } from 'nodeunit';
 import * as YAML from 'yaml';
 import * as eks from '../lib';
@@ -23,7 +22,7 @@ const CLUSTER_VERSION = eks.KubernetesVersion.V1_18;
 
 export = {
 
-  'throws when a non cdk8s chart is added as cdk8s chart'(test: Test) {
+  'throws when a non cdk8s chart construct is added as cdk8s chart'(test: Test) {
 
     const { stack } = testFixture();
 
@@ -39,6 +38,22 @@ export = {
 
   },
 
+  'throws when a core construct is added as cdk8s chart'(test: Test) {
+
+    const { stack } = testFixture();
+
+    const cluster = new eks.Cluster(stack, 'Cluster', {
+      version: CLUSTER_VERSION,
+    });
+
+    // create a plain construct, not a cdk8s chart
+    const someConstruct = new cdk.Construct(stack, 'SomeConstruct');
+
+    test.throws(() => cluster.addCdk8sChart('chart', someConstruct), /Invalid cdk8s chart. Must contain a \'toJson\' method, but found undefined/);
+    test.done();
+
+  },
+
   'cdk8s chart can be added to cluster'(test: Test) {
 
     const { stack } = testFixture();