Support user generated custom native JS classes.

tests/compiler/dart2js_extra/js_interop_error_test.dart

Index: tests/compiler/dart2js_extra/js_interop_error_test.dart
diff --git a/tests/language/vm/regress_23238_test.dart b/tests/compiler/dart2js_extra/js_interop_error_test.dart
similarity index 54%
copy from tests/language/vm/regress_23238_test.dart
copy to tests/compiler/dart2js_extra/js_interop_error_test.dart
index 88250b2a9a924295387efae3bf3b086624d69668..3a94472cb7c2d5c42894ba826797d0d327758847 100644
--- a/tests/language/vm/regress_23238_test.dart
+++ b/tests/compiler/dart2js_extra/js_interop_error_test.dart
@@ -3,13 +3,24 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
+import "package:js/js.dart";
+
+class Bar { }
+
+@Js()
+class Foo extends Bar {
+  external get bar;
+}
+
+@Js()
+class Baz {
+  Baz() {}
+  external get foo;
+  void bar() { }
+  get baz;
+}
 
 void main() {
-  int x = 327680;
-  int r = 65536;
-  for (var i = 0; i < 200; i++) {
-    Expect.equals(r, x ~/ 5);
-    x *= 10;
-    r *= 10;
-  }
+  /// 01: compile-time error
+  var b = new Bar();
 }