[kernel] Fix bug with redirecting constructors in closure conversion.

pkg/kernel/testcases/closures/contexts_in_field_initializers.dart

 // Copyright (c) 2017, 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.
 
 // The purpose of this test is to detect that no unnecessary contexts are
 // created when a constructor parameter is used in its field initializers.  No
 // contexts should be created either in the initializer or in the constructor
 // body.
 
 class X {}
 
 class A {
   X x;
   A(this.x) {}
 }
 
+class B {
+  X x;
+  B(X x) : x = x {
+    fn() {
+      print(x);
+    }
+
+    fn();
+  }
+}
+
 main() {
   A a = new A(new X());
+  B b = new B(new X());
 }