Allow enabling --causal-async-stacks in AOT.

tests/standalone/causal_async_stack_test.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.
+// VMOptions=--causal_async_stacks
+
+import "package:expect/expect.dart";
+
+baz() async {
+  throw "Bad!";
+}
+
+bar() async {
+  await baz();
+}
+
+foo() async {
+  await bar();
+}
+
+main() async {
+  try {
+    await foo();
+  } catch (e, st) {
+    Expect.isTrue(st.toString().contains("baz"));
+    Expect.isTrue(st.toString().contains("bar"));
+    Expect.isTrue(st.toString().contains("foo"));
+    Expect.isTrue(st.toString().contains("main"));
+  }
+}