feat(ElementHandle): add ElementHandle.$ and ElementHandle.$$ (#1151)

This patch adds `ElementHandle.$` and `ElementHandle.$$` methods to query nested
elements.

Fixes #508

lib/ElementHandle.js

@@ -147,6 +147,42 @@ class ElementHandle extends JSHandle {
       clip: boundingBox
     }, options));
   }
+
+  /**
+   * @param {string} selector
+   * @return {!Promise<?ElementHandle>}
+   */
+  async $(selector) {
+    const handle = await this.executionContext().evaluateHandle(
+        (element, selector) => element.querySelector(selector),
+        this, selector
+    );
+    const element = handle.asElement();
+    if (element)
+      return element;
+    await handle.dispose();
+    return null;
+  }
+
+  /**
+   * @param {string} selector
+   * @return {!Promise<!Array<!ElementHandle>>}
+   */
+  async $$(selector) {
+    const arrayHandle = await this.executionContext().evaluateHandle(
+        (element, selector) => element.querySelectorAll(selector),
+        this, selector
+    );
+    const properties = await arrayHandle.getProperties();
+    await arrayHandle.dispose();
+    const result = [];
+    for (const property of properties.values()) {
+      const elementHandle = property.asElement();
+      if (elementHandle)
+        result.push(elementHandle);
+    }
+    return result;
+  }
 }
 
 module.exports = ElementHandle;