fix(assets): can now use multiple assets in the same stack

Fixed by attaching the imported bucket to the Asset object instead
of the Asset object's parent.

Fixes #706.

packages/@aws-cdk/assets/lib/asset.ts

@@ -99,7 +99,7 @@ export class Asset extends cdk.Construct {
         const s3Filename = new cdk.FnSelect(1, new cdk.FnSplit(cxapi.ASSET_PREFIX_SEPARATOR, keyParam.value));
         this.s3ObjectKey = new s3.ObjectKey(new cdk.FnConcat(this.s3Prefix, s3Filename));
 
-        this.bucket = s3.BucketRef.import(parent, 'AssetBucket', {
+        this.bucket = s3.BucketRef.import(this, 'AssetBucket', {
             bucketName: this.s3BucketName
         });