Allow surrogates in string literals.

pkg/compiler/lib/src/string_validator.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.
 
 // Check the validity of string literals.
 
 library stringvalidator;
 
 import 'dart:collection';
 
@@ skipping 182 matching lines @@
               value = value * 16 + hexDigitValue(code);
             }
           }
           code = value;
         }
       }
       if (code >= 0x10000) {
         length++;
         if (code > 0x10FFFF) {
           stringParseError("Invalid code point", token, index);
+          return null;
         }
       }
     }
     // String literal successfully validated.
     if (quoting.raw || !containsEscape) {
       // A string without escapes could just as well have been raw.
       return new DartString.rawString(string, length);
     }
     return new DartString.escapedString(string, length);
   }
 }