VM: Allow trailing comma in assert statements.

tests/language/assert_trailing_comma_test.dart

Index: tests/language/assert_trailing_comma_test.dart
diff --git a/tests/language/assert_trailing_comma_test.dart b/tests/language/assert_trailing_comma_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e0020312e186445ea8379d456ef0631cd8252b38
--- /dev/null
+++ b/tests/language/assert_trailing_comma_test.dart
@@ -0,0 +1,12 @@
+// 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.
+
+main() {
+  assert(true);
+  assert(true,);
+  assert(true,"message");
+  assert(true,"message",);
+  assert(true,"message",extra);  //# 01: compile-time error
+  assert(true,"message",,);  //# 02: compile-time error
+}