feat(core): node.defaultChild as a shortcut to escape hatch (#2684)

Convenience property which returns the child with id "Resource"
or "Default" (fails if there are both).

This is useful in order to enable escape hatching.

Closes #2290

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

@@ -183,6 +183,21 @@ export class ConstructNode {
     return ret;
   }
 
+  /**
+   * Returns the child construct that has the id "Default" or "Resource".
+   * @throws if there is more than one child
+   * @returns a construct or undefined if there is no default child
+   */
+  public get defaultChild(): IConstruct | undefined {
+    const resourceChild = this.tryFindChild('Resource');
+    const defaultChild = this.tryFindChild('Default');
+    if (resourceChild && defaultChild) {
+      throw new Error(`Cannot determine default child for ${this.path}. There is both a child with id "Resource" and id "Default"`);
+    }
+
+    return defaultChild || resourceChild;
+  }
+
   /**
    * All direct children of this construct.
    */

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

@@ -479,6 +479,47 @@ export = {
     test.ok(child2.node.root === root);
     test.ok(child1_1_1.node.root === root);
     test.done();
+  },
+
+  'defaultChild': {
+    'returns the child with id "Resource"'(test: Test) {
+      const root = new Root();
+      new Construct(root, 'child1');
+      const defaultChild = new Construct(root, 'Resource');
+      new Construct(root, 'child2');
+
+      test.same(root.node.defaultChild, defaultChild);
+      test.done();
+    },
+    'returns the child with id "Default"'(test: Test) {
+      const root = new Root();
+      new Construct(root, 'child1');
+      const defaultChild = new Construct(root, 'Default');
+      new Construct(root, 'child2');
+
+      test.same(root.node.defaultChild, defaultChild);
+      test.done();
+    },
+    'returns "undefined" if there is no default'(test: Test) {
+      const root = new Root();
+      new Construct(root, 'child1');
+      new Construct(root, 'child2');
+
+      test.equal(root.node.defaultChild, undefined);
+      test.done();
+    },
+    'fails if there are both "Resource" and "Default"'(test: Test) {
+      const root = new Root();
+      new Construct(root, 'child1');
+      new Construct(root, 'Default');
+      new Construct(root, 'child2');
+      new Construct(root, 'Resource');
+
+      test.throws(() => root.node.defaultChild,
+        /Cannot determine default child for . There is both a child with id "Resource" and id "Default"/);
+      test.done();
+
+    }
   }
 };