Compute the correct signature for default constructors.

dart/tests/language/crash_12118_test.dart

 // Copyright (c) 2013, 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.
 
-library hi_test;
+// Regression test for issue 12118 which caused a crash in dart2js.
 
-import '../../../hi/hi.dart' as hi;
+const X = 42;
 
-// @static-clean
-
-/**
- * This test exists to ensure that the hi sample compiles without errors.
- */
-void main() {
-  
+class A {
+  final x;
+  A({this.x: X});
 }
 
+class B extends A {}
+
+void main() {
+  if (new B().x != 42) {
+    throw 'Test failed';
+  }
+}