Fix type masks for js-interop types.

tests/html/js_typed_interop_type2_test.dart

+import 'dart:html';
+import 'package:js/js.dart';
+import 'package:expect/expect.dart';
+
+@JS()
+@anonymous
+class C {
+  final foo;
+
+  external factory C({foo});
+}
+
+@JS()
+@anonymous
+class D {
+  final foo;
+
+  external factory D({foo});
+}
+
+class F {
+  final foo;
+
+  F(this.foo);
+}
+
+@NoInline()
+testA(A o) {
+  return o.foo;
+}
+
+@NoInline()
+testC(C o) {
+  return o.foo;
+}
+
+@NoInline()
+testF(F o) {
+  return o.foo;
+}
+
+void expectValueOrTypeError(f(), value) {
+  try {
+    String i = 0; // Test for checked mode.
+    Expect.equals(f(), value);
+  } on TypeError catch (error) {
+    Expect.throws(f, (ex) => ex is TypeError);
+  }
+}
+
+main() {
+  var d = new D(foo: 4);
+  var f = new F(6);
+  Expect.equals(testC(d), 4);
+  Expect.equals(testF(f), 6); /// 01: ok
+}
+
+