[es6] Make sure we call add property when adding a new property

When setting a property using `super.prop = val` we need to use
add property if we are adding a new property and not set property.

BUG=493566
LOG=N
R=verwaest@chromium.org, dslomov@chromium.org

Review URL: https://codereview.chromium.org/1161073002

Cr-Commit-Position: refs/heads/master@{#28971}

test/mjsunit/es6/regress/regress-cr493566.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
+
+"use strict";
+
+
+var global = this;
+
+
+(function TestGlobalReceiver() {
+  class A {
+    s() {
+      super.bla = 10;
+    }
+  }
+  new A().s.call(global);
+  assertEquals(10, global.bla);
+})();
+
+
+(function TestProxyProto() {
+  var calls = 0;
+  var handler = {
+    getPropertyDescriptor: function(name) {
+      calls++;
+      return undefined;
+    }
+  };
+
+  var proto = {};
+  var proxy = Proxy.create(handler, proto);
+  var object = {
+    __proto__: proxy,
+    setX(v) {
+      super.x = v;
+    },
+    setSymbol(sym, v) {
+      super[sym] = v;
+    }
+  };
+
+  object.setX(1);
+  assertEquals(1, Object.getOwnPropertyDescriptor(object, 'x').value);
+  assertEquals(1, calls);
+
+  var sym = Symbol();
+  object.setSymbol.call(global, sym, 2);
+  assertEquals(2, Object.getOwnPropertyDescriptor(global, sym).value);
+  // We currently do not invoke proxy traps for symbols
+  assertEquals(1, calls);
+})();
+
+
+(function TestProxyReceiver() {
+  var object = {
+    setY(v) {
+      super.y = v;
+    }
+  };
+
+  var calls = 0;
+  var handler = {
+    getPropertyDescriptor(name) {
+      assertUnreachable();
+    },
+    set(receiver, name, value) {
+      calls++;
+      assertEquals(proxy, receiver);
+      assertEquals('y', name);
+      assertEquals(3, value);
+    }
+  };
+
+  var proxy = Proxy.create(handler);
+  object.setY.call(proxy, 3);
+  assertEquals(1, calls);
+})();