Reduce foot-print of Error.safeToString on strings.

tests/corelib/safe_to_string_test.dart

 // Copyright (c) 2012, 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";
 
 main() {
   Expect.stringEquals('1', Error.safeToString(1));
   Expect.stringEquals('0.5', Error.safeToString(0.5));
   Expect.stringEquals('"1"', Error.safeToString("1"));
   Expect.stringEquals('"\'"', Error.safeToString("'"));
   Expect.stringEquals('"\'\'"', Error.safeToString("''"));
   Expect.stringEquals(r'"\""', Error.safeToString('"'));
   Expect.stringEquals(r'"\"\""', Error.safeToString('""'));
 
   Expect.stringEquals(r'"\\\"\n\r"', Error.safeToString('\\"\n\r'));
 
-  Expect.stringEquals(r'"\x00\x01\x02\x03\x04\x05\x06\x07"',
+  Expect.stringEquals(r'"\u0000\u0001\u0002\u0003\u0004\u0005\u0006\u0007"',
                       Error.safeToString('\x00\x01\x02\x03\x04\x05\x06\x07'));
-  Expect.stringEquals(r'"\x08\t\n\x0b\x0c\r\x0e\x0f"',
+  Expect.stringEquals(r'"\b\t\n\u000b\f\r\u000e\u000f"',
                       Error.safeToString('\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f'));
-  Expect.stringEquals(r'"\x10\x11\x12\x13\x14\x15\x16\x17"',
+  Expect.stringEquals(r'"\u0010\u0011\u0012\u0013\u0014\u0015\u0016\u0017"',
                       Error.safeToString('\x10\x11\x12\x13\x14\x15\x16\x17'));
-  Expect.stringEquals(r'"\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f"',
+  Expect.stringEquals(r'"\u0018\u0019\u001a\u001b\u001c\u001d\u001e\u001f"',
                       Error.safeToString('\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f'));
   Expect.stringEquals('" "', Error.safeToString(" "));
 
   Expect.stringEquals('null', Error.safeToString(null));
   Expect.stringEquals('true', Error.safeToString(true));
   Expect.stringEquals('false', Error.safeToString(false));
   Expect.stringEquals("Instance of 'Object'",
                       Error.safeToString(new Object()));
 }