Add some support for the code-point code-unit distinction.

sdk/lib/_internal/compiler/implementation/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 "dart2jslib.dart";
 import "tree/tree.dart";
@@ skipping 150 matching lines @@
                 stringParseError("Invalid character in escape sequence",
                                  token, index);
                 return null;
               }
               value = value * 16 + hexDigitValue(code);
             }
           }
           code = value;
         }
       }
+      // TODO(erikcorry): Use the string code point iterator when available.
+      if (code >= String.SMP_CODE_POINT_BASE) ++length;;
       // This handles both unescaped characters and the value of unicode
       // escapes.
       if (!isUnicodeScalarValue(code)) {
         stringParseError(
             "Invalid Unicode scalar value U+${code.toRadixString(16)}",
             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);
   }
 }