Fix several bugs in closure conversion.

pkg/kernel/testcases/closures/arity.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.
+//
+// Test that closures check their argument count.
+//
+main() {
+  var closures = [
+    (x, y, [z]) {},
+    (x, y, z) {},
+    (x, y, {z}) {},
+    (x, y, z, w, v) {}
+  ];
+  for (var c in closures) {
+    bool ok = false;
+    try {
+      c(1, 2, 3, 4);
+    } on NoSuchMethodError catch (_) {
+      ok = true;
+    }
+    if (!ok) {
+      throw new Exception("Expected an error!");
+    }
+  }
+
+  (x, y, [z]) {}(1, 2);
+  (x, y, [z]) {}(1, 2, 3);
+}