[proxies] Implement Symbol/DONT_ENUM filtering for GetKeys()

test/mjsunit/harmony/proxies-keys.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-proxies
+
+var target = {
+  target: 1
+};
+target.__proto__ = {
+  target_proto: 2
+};
+
+var handler = {
+  ownKeys: function(target) {
+    return ["foo", "bar", Symbol("baz"), "non-enum", "not-found"];
+  },
+  getOwnPropertyDescriptor: function(target, name) {
+    if (name == "non-enum") return {configurable: true};
+    if (name == "not-found") return undefined;
+    return {enumerable: true, configurable: true};
+  }
+}
+
+var proxy = new Proxy(target, handler);
+
+// Object.keys() ignores symbols and non-enumerable keys.
+assertEquals(["foo", "bar"], Object.keys(proxy));
+
+// Edge case: no properties left after filtering.
+handler.getOwnPropertyDescriptor = undefined;
+assertEquals([], Object.keys(proxy));
+
+// Throwing shouldn't crash.
+handler.getOwnPropertyDescriptor = function() { throw new Number(1); };
+assertThrows("Object.keys(proxy)", Number);
+
+// Fall through to target if there is no trap.
+handler.ownKeys = undefined;
+assertEquals(["target"], Object.keys(proxy));
+assertEquals(["target"], Object.keys(target));