fix(apigateway): authorizer is not attached to RestApi across projects

packages/@aws-cdk/aws-apigateway/lib/authorizer.ts

@@ -1,14 +1,29 @@
-import { Resource } from '@aws-cdk/core';
+import { Construct, Resource, ResourceProps } from '@aws-cdk/core';
 import { AuthorizationType } from './method';
 import { RestApi } from './restapi';
 
+const AUTHORIZER_SYMBOL = Symbol.for('@aws-cdk/aws-apigateway.Authorizer');
+
 /**
  * Base class for all custom authorizers
  */
 export abstract class Authorizer extends Resource implements IAuthorizer {
+  /**
+   * Return whether the given object is an Authorizer.
+   */
+  public static isAuthorizer(x: any): x is Authorizer {
+    return x !== null && typeof(x) === 'object' && AUTHORIZER_SYMBOL in x;
+  }
+
   public readonly abstract authorizerId: string;
   public readonly authorizationType?: AuthorizationType = AuthorizationType.CUSTOM;
 
+  public constructor(scope: Construct, id: string, props?: ResourceProps) {
+    super(scope, id, props);
+
+    Object.defineProperty(this, AUTHORIZER_SYMBOL, { value: true });
+  }
+
   /**
    * Called when the authorizer is used from a specific REST API.
    * @internal

packages/@aws-cdk/aws-apigateway/lib/method.ts

@@ -185,7 +185,7 @@ export class Method extends Resource {
         `which is different from what is required by the authorizer [${authorizer.authorizationType}]`);
     }
 
-    if (authorizer instanceof Authorizer) {
+    if (Authorizer.isAuthorizer(authorizer)) {
       authorizer._attachToApi(this.restApi);
     }
 

packages/@aws-cdk/aws-apigateway/test/test.authorizer.ts

@@ -0,0 +1,21 @@
+import { Stack } from '@aws-cdk/core';
+import { Test } from 'nodeunit';
+import { Authorizer, RestApi } from '../lib';
+
+export = {
+  'isAuthorizer correctly detects an instance of type Authorizer'(test: Test) {
+    class MyAuthorizer extends Authorizer {
+      public readonly authorizerId = 'test-authorizer-id';
+      public _attachToApi(_: RestApi): void {
+        // do nothing
+      }
+    }
+    const stack = new Stack();
+    const authorizer = new MyAuthorizer(stack, 'authorizer');
+
+    test.ok(Authorizer.isAuthorizer(authorizer), 'type Authorizer expected but is not');
+    test.ok(!Authorizer.isAuthorizer(stack), 'type Authorizer found, when not expected');
+
+    test.done();
+  },
+};