Handle BOMs in dart2js.

tests/language/unicode_bom_test.dart

Index: tests/language/unicode_bom_test.dart
diff --git a/tests/language/unicode_bom_test.dart b/tests/language/unicode_bom_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..2e234458d9a812adf5b88680a98f767bb7efccbd
--- /dev/null
+++ b/tests/language/unicode_bom_test.dart
@@ -0,0 +1,11 @@
+// Copyright (c) 2011, 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 file is saved with a BOM character (as first character). This character
+// should be ignored.
+// Tests that files with a BOM character are correctly handled.
+
+main() {
+  /* do nothing. */
+}