Do not add forwarding constructors to a pricate constructor from a different library.

tests/language/regress_20394_test.dart

Index: tests/language/regress_20394_test.dart
diff --git a/tests/language/regress_20394_test.dart b/tests/language/regress_20394_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..67a65f7683c7fb46d50e4735808963a21f5f4fc2
--- /dev/null
+++ b/tests/language/regress_20394_test.dart
@@ -0,0 +1,11 @@
+import 'regress_20394_lib.dart';
+
+class M {}
+
+class C extends Super with M {
+  C() : super._private(42);   /// 01: compile-time error
+}
+
+main() {
+  new C();
+}