new classes: change semantics of super(...) call and add new.target to construct stub.

test/mjsunit/harmony/classes-experimental.js

+// Copyright 2014 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: --experimental-classes --harmony-classes
+
+'use strict';
+
+class Base {
+  constructor() {
+    let o = new Object();
+    o.prp = 1;
+    return o;
+  }
+}
+
+class Subclass extends Base {
+  constructor() {
+    try {
+      this.prp1 = 3;
+    } catch (e) {
+      // TODO(dslomov): actually test the exception once TDZ is implemented.
+    }
+    super();
+    assertSame(1, this.prp);
+    assertSame(undefined, this.prp1);
+    assertFalse(this.hasOwnProperty("prp1"));
+    return this;
+  }
+}
+
+let s = new Subclass();
+assertSame(1, s.prp);
+assertSame(undefined, s.prp1);
+assertFalse(s.hasOwnProperty("prp1"));