Remove unnecessary contexts 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) {}
+}
+
+main() {
+  A a = new A(new X());
+}