MAGETWO-39446: "Place Order" button should be disabled until billing address is set

app/code/Magento/Checkout/view/frontend/web/js/view/billing-address.js

@@ -39,7 +39,7 @@ define(
                 this._super()
                     .observe({
                         selectedAddress: null,
-                        isAddressDetailsVisible: quote.shippingAddress() != null,
+                        isAddressDetailsVisible: quote.billingAddress() != null,
                         isAddressFormVisible: !customer.isLoggedIn() || addressOptions.length == 1,
                         isAddressSameAsShipping: false
                     });

app/code/Magento/Checkout/view/frontend/web/js/view/payment/default.js

@@ -17,7 +17,7 @@ define(
         'use strict';
         return Component.extend({
             redirectAfterPlaceOrder: true,
-            isPlaceOrderActionAllowed: ko.observable(true),
+            isPlaceOrderActionAllowed: ko.observable(quote.billingAddress() != null),
             /**
              * Initialize view.
              *