Preserve the contents of Dart strings with unmatched surrogate halfs by avoiding a UTF16 -> UTF8

runtime/bin/vmservice/observatory/lib/src/elements/observatory_element.dart

 // Copyright (c) 2013, 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.
 
 library observatory_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:observatory/app.dart';
 import 'package:polymer/polymer.dart';
@@ skipping 107 matching lines @@
     for (int codeUnit in value.codeUnits) {
       if (codeUnit == '\n'.codeUnitAt(0)) result.addAll('\\n'.codeUnits);
       else if (codeUnit == '\r'.codeUnitAt(0)) result.addAll('\\r'.codeUnits);
       else if (codeUnit == '\f'.codeUnitAt(0)) result.addAll('\\f'.codeUnits);
       else if (codeUnit == '\b'.codeUnitAt(0)) result.addAll('\\b'.codeUnits);
       else if (codeUnit == '\t'.codeUnitAt(0)) result.addAll('\\t'.codeUnits);
       else if (codeUnit == '\v'.codeUnitAt(0)) result.addAll('\\v'.codeUnits);
       else if (codeUnit == '\$'.codeUnitAt(0)) result.addAll('\\\$'.codeUnits);
       else if (codeUnit == '\\'.codeUnitAt(0)) result.addAll('\\\\'.codeUnits);
       else if (codeUnit == "'".codeUnitAt(0)) result.addAll("'".codeUnits);
-      else if (codeUnit < 32) result.addAll("\\u$codeUnit".codeUnits);
-      else result.add(codeUnit);
+      else if (codeUnit < 32) {
+         var escapeSequence = "\\u" + codeUnit.toRadixString(16).padLeft(4, "0");
+         result.addAll(escapeSequence.codeUnits);
+      } else result.add(codeUnit);
     }
     if (wasTruncated) {
       result.addAll("...".codeUnits);
     } else {
       result.add("'".codeUnitAt(0));
     }
     return new String.fromCharCodes(result);
   }
 }