Fix several bugs in closure conversion.

pkg/kernel/testcases/closures/syncstar.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.
+//
+// This tests that the wrapper function around converted closures in the VM
+// doesn't break when the context parameter is captured (since async
+// transformations introduces an additional closure here).
+
+range(int high) {
+  iter(int low) sync* {
+    while (high-- > low) yield high;
+  }
+
+  return iter;
+}
+
+main() {
+  var sum = 0;
+  for (var x in range(10)(2)) sum += x;
+
+  if (sum != 44) {
+    throw new Exception("Incorrect output.");
+  }
+}