[IMP] component: defaultProps application

We re-add the application of defaultProps. Note that the application
is done twice in dev mode.

src/component/props_validation.ts

@@ -1,5 +1,22 @@
 import { Component } from "./component";
 
+/**
+ * Apply default props (only top level).
+ *
+ * Note that this method does modify in place the props
+ */
+export function applyDefaultProps(props: { [key: string]: any }, ComponentClass: typeof Component) {
+  const defaultProps = (ComponentClass as any).defaultProps
+  if (defaultProps) {
+    for (let propName in defaultProps) {
+      if (props![propName] === undefined) {
+        props![propName] = defaultProps[propName];
+      }
+    }
+  }
+}
+
+
 //------------------------------------------------------------------------------
 // Prop validation helper
 //------------------------------------------------------------------------------
@@ -17,6 +34,8 @@ export const validateProps = function (name: string | typeof Component, props: a
   const ComponentClass =
     typeof name !== "string" ? name : parent.constructor.components[name as string];
 
+  applyDefaultProps(props, ComponentClass);
+
   const propsDef = (<any>ComponentClass).props;
   if (propsDef instanceof Array) {
     // list of strings (prop names)