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

tests/compiler/dart2js_extra/4740_int_library.dart

Index: tests/compiler/dart2js_extra/4740_int_library.dart
diff --git a/tests/compiler/dart2js_extra/regress/4740_library.dart b/tests/compiler/dart2js_extra/4740_int_library.dart
similarity index 64%
copy from tests/compiler/dart2js_extra/regress/4740_library.dart
copy to tests/compiler/dart2js_extra/4740_int_library.dart
index fd51e977bfed4d5d1d682e46da3b72af4e5056a3..17b506df4682558849df4efca19998b121d8b0fc 100644
--- a/tests/compiler/dart2js_extra/regress/4740_library.dart
+++ b/tests/compiler/dart2js_extra/4740_int_library.dart
@@ -2,8 +2,12 @@
 // 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.
 
-#library('4740_library');
+#library('4740_int_library');
 
-class Foo {
+class Foo implements FooInterface {
   Foo._internal();
 }
+
+interface FooInterface default Foo {
+  FooInterface._internal();
+}