fix(cdk): don't use instanceof in App

Make the Stack instance test no longer use instanceof, which doesn't
work properly in case of multiple installed copies of the library.

This does not resolve but helps with #1245.

packages/@aws-cdk/cdk/lib/app.ts

@@ -1,7 +1,7 @@
 import cxapi = require('@aws-cdk/cx-api');
 import fs = require('fs');
 import path = require('path');
-import { Stack } from './cloudformation/stack';
+import { isStack, Stack } from './cloudformation/stack';
 import { Construct, MetadataEntry, PATH_SEP, Root } from './core/construct';
 import { resolve } from './core/tokens';
 
@@ -21,8 +21,8 @@ export class App extends Root {
   private get stacks() {
     const out: { [name: string]: Stack } = { };
     for (const child of this.children) {
-      if (!(child instanceof Stack)) {
-        throw new Error(`The child ${child.toString()} of Program must be a Stack`);
+      if (!isStack(child)) {
+        throw new Error(`The child ${child.toString()} of App must be a Stack`);
       }
 
       out[child.id] = child as Stack;

packages/@aws-cdk/cdk/lib/cloudformation/stack.ts

@@ -438,7 +438,7 @@ export abstract class Referenceable extends StackElement {
  *
  * We do attribute detection since we can't reliably use 'instanceof'.
  */
-function isStack(construct: Construct): construct is Stack {
+export function isStack(construct: Construct): construct is Stack {
   return (construct as any).isStack;
 }