Handle BOMs in dart2js.

tests/language/unicode_bom_middle_test.dart

Index: tests/language/unicode_bom_middle_test.dart
diff --git a/tests/language/unicode_bom_middle_test.dart b/tests/language/unicode_bom_middle_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..788807f778c3180deff9e38b47ca77cf98444d46
--- /dev/null
+++ b/tests/language/unicode_bom_middle_test.dart
@@ -0,0 +1,17 @@
+// 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.
+
+import "package:expect/expect.dart";
+
+int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
+
+foo(x) {
+  if (inscrutable(1999) == 1999) return x;
+  return 499;
+}
+
+main() {
+  Expect.equals(3, "xx".length);  // BOM character between the xs.
+  Expect.equals(3, foo("xx").length);  // BOM character between the xs.
+}