closure conversion: Support closures in initializers

pkg/kernel/testcases/closures/closure_in_initializer_closure.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.md file.
+
+class C {
+  var t;
+  C.foo(f) : t = (() {
+      var prefix;
+      var g = (x) {
+        f("$prefix$x");
+      };
+      prefix = 'hest';
+      return g;
+    }) {
+    print(1);
+  }
+}
+
+main() {
+  print(0);
+  var c = new C.foo((x) => print(x));
+  print(2);
+  c.t()('fisk');
+  print(3);
+}