Reverse dependency edge between aws-sns and aws-ec2 packages

This is because otherwise the new dependency of aws-lambda => aws-ec2
would introduce the following cycle:

Cycle: @aws-cdk/aws-ec2 => @aws-cdk/aws-sns => @aws-cdk/aws-lambda =>
@aws-cdk/aws-ec2

packages/@aws-cdk/aws-ec2/lib/auto-scaling-group.ts

@@ -1,6 +1,5 @@
 import autoscaling = require('@aws-cdk/aws-autoscaling');
 import iam = require('@aws-cdk/aws-iam');
-import sns = require('@aws-cdk/aws-sns');
 import cdk = require('@aws-cdk/cdk');
 import { Connections, IConnectable } from './connections';
 import { InstanceType } from './instance-types';
@@ -62,7 +61,7 @@ export interface AutoScalingGroupProps {
      * SNS topic to send notifications about fleet changes
      * @default No fleet change notifications will be sent.
      */
-    notificationsTopic?: sns.cloudformation.TopicResource;
+    notificationsTopic?: IAutoScalingNotificationTarget;
 
     /**
      * Whether the instances can initiate connections to anywhere by default
@@ -152,7 +151,7 @@ export class AutoScalingGroup extends cdk.Construct implements IClassicLoadBalan
         if (props.notificationsTopic) {
             asgProps.notificationConfigurations = [];
             asgProps.notificationConfigurations.push({
-                topicArn: props.notificationsTopic.ref,
+                topicArn: props.notificationsTopic.asAutoScalingNotificationTarget(),
                 notificationTypes: [
                     "autoscaling:EC2_INSTANCE_LAUNCH",
                     "autoscaling:EC2_INSTANCE_LAUNCH_ERROR",
@@ -192,3 +191,19 @@ export class AutoScalingGroup extends cdk.Construct implements IClassicLoadBalan
         this.role.addToPolicy(statement);
     }
 }
+
+/**
+ * Interface for subscribing an SNS topic to AutoScalingGroup notifications
+ *
+ * (This interface exists to reverse the dependency between the aws-sns
+ * and aws-ec2 packages.)
+ */
+export interface IAutoScalingNotificationTarget {
+    /**
+     * ARN of the topic to send notifications to.
+     */
+    asAutoScalingNotificationTarget(): cdk.Arn;
+
+    // NOTE: this cannot just be "topicArn: Arn" because of lack of return type
+    // covariance in jsii.
+}

packages/@aws-cdk/aws-ec2/package.json

@@ -56,7 +56,6 @@
     "@aws-cdk/aws-autoscaling": "^0.8.2",
     "@aws-cdk/aws-elasticloadbalancing": "^0.8.2",
     "@aws-cdk/aws-iam": "^0.8.2",
-    "@aws-cdk/aws-sns": "^0.8.2",
     "@aws-cdk/cdk": "^0.8.2",
     "@aws-cdk/util": "^0.8.2"
   },

packages/@aws-cdk/aws-sns/lib/topic-ref.ts

@@ -1,4 +1,5 @@
 import cloudwatch = require('@aws-cdk/aws-cloudwatch');
+import ec2 = require('@aws-cdk/aws-ec2');
 import events = require('@aws-cdk/aws-events');
 import iam = require('@aws-cdk/aws-iam');
 import lambda = require('@aws-cdk/aws-lambda');
@@ -17,7 +18,9 @@ export class TopicArn extends cdk.Arn { }
 /**
  * Either a new or imported Topic
  */
-export abstract class TopicRef extends cdk.Construct implements events.IEventRuleTarget, cloudwatch.IAlarmAction, s3n.IBucketNotificationDestination {
+export abstract class TopicRef extends cdk.Construct implements events.IEventRuleTarget, cloudwatch.IAlarmAction, s3n.IBucketNotificationDestination,
+        ec2.IAutoScalingNotificationTarget {
+
     /**
      * Import a Topic defined elsewhere
      */
@@ -306,6 +309,10 @@ export abstract class TopicRef extends cdk.Construct implements events.IEventRul
             dependencies: [ this.policy! ] // make sure the topic policy resource is created before the notification config
         };
     }
+
+    public asAutoScalingNotificationTarget(): cdk.Arn {
+        return this.topicArn;
+    }
 }
 
 /**

packages/@aws-cdk/aws-sns/package.json

@@ -55,6 +55,7 @@
   },
   "dependencies": {
     "@aws-cdk/aws-cloudwatch": "^0.8.2",
+    "@aws-cdk/aws-ec2": "^0.8.2",
     "@aws-cdk/aws-events": "^0.8.2",
     "@aws-cdk/aws-iam": "^0.8.2",
     "@aws-cdk/aws-lambda": "^0.8.2",