Add ES2015 Proxies sample (#275)

ES2015 Proxies sample

proxies-es6/README.md

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+
+Proxies (ES2015) Sample
+===
+See https://googlechrome.github.io/samples/proxies-es6/index.html for a live demo.
+
+Learn more at https://www.chromestatus.com/feature/4811188005240832

proxies-es6/demo.js

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+'use strict';
+
+// Proxying a normal object
+var target = {};
+var handler = {
+  get(target, property, receiver) { // eslint-disable-line no-unused-vars
+    return `Hello, ${property}!`;
+  }
+};
+
+var myProxy = new Proxy(target, handler);
+ChromeSamples.log(myProxy.folks);
+// myProxy.folks would normally be undefined, but the proxied get handler provides a value.
+// Outputs: Hello, folks!
+
+// Proxying a function object
+function sum(a, b) {
+  return a + b;
+}
+
+var handler2 = {
+  apply(target, thisArg, argumentsList) {
+    ChromeSamples.log(`Calculate sum: ${argumentsList}`);
+    return target.apply(thisArg, argumentsList);
+  }
+};
+
+var proxy = new Proxy(sum, handler2);
+ChromeSamples.log(proxy(1, 2));
+// Calculate sum: 1,2
+// 3
+
+// Field interception with Proxy and the Reflect object.
+//
+// Reflect is a built-in object that provides methods for interceptable
+// JavaScript operations. The methods are the same as those of proxy
+// handlers. Reflect is not a function object, so it's not constructible.
+
+var pioneer = new Proxy({}, {
+  get(target, property, receiver) {
+    ChromeSamples.log(`get called for field: ${property}`);
+    return Reflect.get(target, property, receiver);
+  },
+  set(target, property, value, receiver) {
+    ChromeSamples.log(`set called for field: ${property} and value: ${value}`);
+    return Reflect.set(target, property, value, receiver);
+  }
+});
+
+pioneer.firstName = 'Grace';
+pioneer.secondName = 'Hopper';
+

proxies-es6/index.html

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+---
+feature_name: Proxies (ES2015)
+chrome_version: 49
+feature_id: 4811188005240832
+---
+
+<h3>Background</h3>
+<p>ES2015 <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Proxy">Proxies</a> enable creation of objects with the full range of behaviors available to host objects. They can be used for interception, object virtualization and logging/profiling.</p>
+
+{% include output_helper.html %}
+
+{% include js_snippet.html filename='demo.js' %}