fix(cloudfront): expose CfnDistribution as defaultChild (#4556)

* fix(cloudfront): export CfnDistribution as defaultChild

* chore: fix import order

Author: Jimmy Gaussen

packages/@aws-cdk/aws-cloudfront/lib/web_distribution.ts

@@ -697,6 +697,7 @@ export class CloudFrontWebDistribution extends cdk.Construct implements IDistrib
     }
 
     const distribution = new CfnDistribution(this, 'CFDistribution', { distributionConfig });
+    this.node.defaultChild = distribution;
     this.domainName = distribution.attrDomainName;
     this.distributionId = distribution.ref;
   }

packages/@aws-cdk/aws-cloudfront/test/test.basic.ts

@@ -3,7 +3,7 @@ import * as lambda from '@aws-cdk/aws-lambda';
 import s3 = require('@aws-cdk/aws-s3');
 import cdk = require('@aws-cdk/core');
 import { Test } from 'nodeunit';
-import { CloudFrontWebDistribution, LambdaEdgeEventType, ViewerProtocolPolicy } from '../lib';
+import { CfnDistribution, CloudFrontWebDistribution, LambdaEdgeEventType, ViewerProtocolPolicy } from '../lib';
 
 // tslint:disable:object-literal-key-quotes
 
@@ -456,4 +456,23 @@ export = {
     test.done();
   },
 
+  'distribution has a defaultChild'(test: Test) {
+    const stack = new cdk.Stack();
+    const sourceBucket = new s3.Bucket(stack, 'Bucket');
+
+    const distribution = new CloudFrontWebDistribution(stack, 'AnAmazingWebsiteProbably', {
+      originConfigs: [
+        {
+          s3OriginSource: {
+            s3BucketSource: sourceBucket
+          },
+          behaviors: [{ isDefaultBehavior: true }]
+        }
+      ]
+    });
+
+    test.ok(distribution.node.defaultChild instanceof CfnDistribution);
+    test.done();
+  },
+
 };