fix(cdk): don't use instanceof in App (#1249)

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

@@ -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 (!Stack.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

@@ -35,7 +35,7 @@ export class Stack extends Construct {
    */
   public static find(node: Construct): Stack {
     let curr: Construct | undefined = node;
-    while (curr != null && !isStack(curr)) {
+    while (curr != null && !Stack.isStack(curr)) {
       curr = curr.parent;
     }
 
@@ -58,6 +58,15 @@ export class Stack extends Construct {
     construct.addMetadata('aws:cdk:physical-name', physicalName);
   }
 
+  /**
+   * Return whether the given object is a Stack.
+   *
+   * We do attribute detection since we can't reliably use 'instanceof'.
+   */
+  public static isStack(construct: Construct): construct is Stack {
+    return (construct as any)._isStack;
+  }
+
   private static readonly VALID_STACK_NAME_REGEX = /^[A-Za-z][A-Za-z0-9-]*$/;
 
   /**
@@ -72,11 +81,6 @@ export class Stack extends Construct {
    */
   public readonly env: Environment;
 
-  /**
-   * Used to determine if this construct is a stack.
-   */
-  public readonly isStack = true;
-
   /**
    * Logical ID generation strategy
    */
@@ -92,6 +96,11 @@ export class Stack extends Construct {
    */
   public readonly name: string;
 
+  /**
+   * Used to determine if this construct is a stack.
+   */
+  protected readonly _isStack = true;
+
   /**
    * Creates a new stack.
    *
@@ -433,15 +442,6 @@ export abstract class Referenceable extends StackElement {
   }
 }
 
-/**
- * Return whether the given object is a Stack.
- *
- * We do attribute detection since we can't reliably use 'instanceof'.
- */
-function isStack(construct: Construct): construct is Stack {
-  return (construct as any).isStack;
-}
-
 /**
  * Collect all StackElements from a construct
  *

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

@@ -45,8 +45,8 @@ export = {
   'Stack.isStack indicates that a construct is a stack'(test: Test) {
     const stack = new Stack();
     const c = new Construct(stack, 'Construct');
-    test.ok(stack.isStack);
-    test.ok(!(c as any).isStack);
+    test.ok(Stack.isStack(stack));
+    test.ok(!Stack.isStack(c));
     test.done();
   },