Made dart2js constructor lookup logic "private"-aware, fixed 4740 bug.

tests/compiler/dart2js_extra/4740_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#import('4740_library.dart'); /// 01: continued
+#import('4740_library.dart'); /// 02: continued
+#import('4740_int_library.dart'); /// 03: continued
+
+class FooBar extends Foo {
+  FooBar() : super._internal(); /// 01: compile-time error
+}
+
+main() {
+  new FooBar(); /// 01: continued
+  Expect.throws(() => new Foo._internal(), /// 02: static type warning
+                (e) => e is NoSuchMethodError);  /// 02: continued
+  Expect.throws(() => new FooInterface._internal(), /// 03: static type warning
+                (e) => e is NoSuchMethodError);  /// 03: continued
+}