VM: Add missing token position in async rethrow

tests/language/vm/regress_28325_test.dart

Index: tests/language/vm/regress_28325_test.dart
diff --git a/tests/language/vm/regress_28325_test.dart b/tests/language/vm/regress_28325_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..06a922ded4d4f4b0d28953a6ef2e93e37877ee89
--- /dev/null
+++ b/tests/language/vm/regress_28325_test.dart
@@ -0,0 +1,20 @@
+// 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 source positions in async errors.
+
+import "package:expect/expect.dart";
+import "dart:io";
+
+main() async {
+  try {
+    await Socket.connect("localhost", 0);
+    Expect.isTrue(false);  // Unreachable.
+  } catch (e, s) {
+    Expect.isTrue(e is SocketException);
+    Expect.isTrue(s.toString().contains("regress_28325_test.dart"));
+    print(s);
+    Expect.isTrue(s.toString().contains(":12:5"));  // Position of "await".
+  }
+}