fix(aws-sqs): Queue.import() doesn't return a value

`QueueRef.import` did not return the created `ImportedQueue` object.
Added assertions to the relevant test to verify that indeed exports
and imports behave as expected.

Fixes #879

packages/@aws-cdk/aws-sqs/lib/queue-ref.ts

@@ -11,8 +11,8 @@ export abstract class QueueRef extends cdk.Construct implements s3n.IBucketNotif
   /**
    * Import an existing queue
    */
-  public static import(parent: cdk.Construct, name: string, props: QueueRefProps) {
-    new ImportedQueue(parent, name, props);
+  public static import(parent: cdk.Construct, name: string, props: QueueRefProps): QueueRef {
+    return new ImportedQueue(parent, name, props);
   }
 
   /**

packages/@aws-cdk/aws-sqs/test/test.sqs.ts

@@ -91,12 +91,24 @@ export = {
   },
 
   'exporting and importing works'(test: Test) {
+    // GIVEN
     const stack = new Stack();
     const queue = new sqs.Queue(stack, 'Queue');
 
+    // WHEN
     const ref = queue.export();
+    const imports = sqs.QueueRef.import(stack, 'Imported', ref);
 
-    sqs.QueueRef.import(stack, 'Imported', ref);
+    // THEN
+
+    // "import" returns a a QueueRef bound to `Fn::ImportValue`s.
+    test.deepEqual(resolve(imports.queueArn), { 'Fn::ImportValue': 'QueueQueueArn8CF496D5' });
+    test.deepEqual(resolve(imports.queueUrl), { 'Fn::ImportValue': 'QueueQueueUrlC30FF916' });
+
+    // the exporting stack has Outputs for QueueARN and QueueURL
+    const outputs = stack.toCloudFormation().Outputs;
+    test.deepEqual(outputs.QueueQueueArn8CF496D5, { Value: { 'Fn::GetAtt': [ 'Queue4A7E3555', 'Arn' ] }, Export: { Name: 'QueueQueueArn8CF496D5' } });
+    test.deepEqual(outputs.QueueQueueUrlC30FF916, { Value: { Ref: 'Queue4A7E3555' }, Export: { Name: 'QueueQueueUrlC30FF916' } });
 
     test.done();
   },