Don't crash when generating error messages with malformed strings.

tests/corelib/throw_half_surrogate_pair_test.dart

Index: tests/corelib/throw_half_surrogate_pair_test.dart
diff --git a/tests/corelib/throw_half_surrogate_pair_test.dart b/tests/corelib/throw_half_surrogate_pair_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e4a5f0ed1bca79afa5ac50e047454110be920074
--- /dev/null
+++ b/tests/corelib/throw_half_surrogate_pair_test.dart
@@ -0,0 +1,12 @@
+// Copyright (c) 2014, 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() {
+  var trebleClef = "\u{1D11E}";
+  if (trebleClef.length != 2) throw "String should be a surrogate pair";
+  // These uncaught exceptions should not caush the VM to crash attempting to
+  // print a malformed string.
+  throw trebleClef[0];  /// 01: runtime error
+  throw trebleClef[1];  /// 02: runtime error
+}