fix(s3-notifications): cannot use imported lambda ("fromArn") as a destination

packages/@aws-cdk/aws-s3-notifications/lib/lambda.ts

@@ -23,7 +23,7 @@ export class LambdaDestination implements s3.IBucketNotificationDestination {
 
     // if we have a permission resource for this relationship, add it as a dependency
     // to the bucket notifications resource, so it will be created first.
-    const permission = this.fn.node.findChild(permissionId) as CfnResource;
+    const permission = this.fn.node.tryFindChild(permissionId) as CfnResource | undefined;
 
     return {
       type: s3.BucketNotificationDestinationType.LAMBDA,

packages/@aws-cdk/aws-s3-notifications/test/lambda/lambda.test.ts

@@ -43,3 +43,30 @@ test('lambda as notification target', () => {
     }
   });
 });
+
+test('lambda as notification target specified by function arn', () => {
+  // GIVEN
+  const stack = new Stack();
+  const bucketA = new s3.Bucket(stack, 'MyBucket');
+  const fn = lambda.Function.fromFunctionArn(stack, 'MyFunction', 'arn:aws:lambda:us-east-1:123456789012:function:ProcessKinesisRecords');
+
+  // WHEN
+  bucketA.addObjectCreatedNotification(new s3n.LambdaDestination(fn), { suffix: '.png' });
+
+  // THEN
+  expect(stack).toHaveResource('Custom::S3BucketNotifications', {
+    NotificationConfiguration: {
+      LambdaFunctionConfigurations: [
+        {
+          Events: [ "s3:ObjectCreated:*" ],
+          Filter: {
+            Key: {
+              FilterRules: [ { Name: "suffix", Value: ".png" } ]
+            }
+          },
+          LambdaFunctionArn: "arn:aws:lambda:us-east-1:123456789012:function:ProcessKinesisRecords"
+        }
+      ]
+    }
+  });
+});