Update dart2js unit tests to dart_parser and dart_scanner.

tests/compiler/dart2js/scanner_offset_length_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";
-import 'package:compiler/src/scanner/string_scanner.dart';
-import 'package:compiler/src/tokens/token.dart';
-import 'package:compiler/src/tokens/token_constants.dart';
+import 'package:dart_scanner/dart_scanner.dart';
 
 Token scan(String text) =>
-    new StringScanner.fromString(text, includeComments: true).tokenize();
+    new StringScanner(text, includeComments: true).tokenize();
 
 check(String text) {
   Token token = scan(text);
   while (token.kind != EOF_TOKEN) {
     Expect.equals(token.value.length, token.charCount);
 
     var start = token.charOffset;
     var end = token.charOffset + token.charCount;
 
     Expect.isTrue(start < text.length,
@@ skipping 21 matching lines @@
 main() {
   check('foo'); // identifier
   check('\'\''); // empty string
   check('\'foo\''); // simple string
   check('\'\$foo\''); // interpolation, identifier
   check('\'\${foo}\''); // interpolation, expression
 
   check('//'); // single line comment
   check('/**/'); // multi line comment
 }