Fix the StringScanner tests under dart2js.

pkg/string_scanner/lib/string_scanner.dart

 // Copyright (c) 2014, 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.
 
 /// A library for parsing strings using a sequence of patterns.
 library string_scanner;
 
 import 'dart:math' as math;
 
+/// When compiled to JS, forward slashes are always escaped in [RegExp.pattern].
+///
+/// See issue 17998.
+final _slashAutoEscape = new RegExp("/").pattern == "\\/";
+
 // TODO(nweiz): Add some integration between this and source maps.
 /// A class that scans through a string using [Pattern]s.
 class StringScanner {
   /// The string being scanned through.
   final String string;
 
   /// The current position of the scanner in the string, in characters.
   int get position => _position;
   set position(int position) {
     if (position < 0 || position > string.length) {
@@ skipping 38 matching lines @@
   ///
   /// If [pattern] did not match, throws a [FormatException] describing the
   /// position of the failure. [name] is used in this error as the expected name
   /// of the pattern being matched; if it's `null`, the pattern itself is used
   /// instead.
   void expect(Pattern pattern, {String name}) {
     if (scan(pattern)) return;
 
     if (name == null) {
       if (pattern is RegExp) {
-        name = "/${pattern.pattern.replaceAll("/", "\\/")}/";
+        var source = pattern.pattern;
+        if (!_slashAutoEscape) source = source.replaceAll("/", "\\/");
+        name = "/$source/";
       } else {
         name = pattern.toString()
             .replaceAll("\\", "\\\\").replaceAll('"', '\\"');
         name = '"$name"';
       }
     }
     _fail(name);
   }
 
   /// If the string has not been fully consumed, this throws a
@@ skipping 74 matching lines @@
         "$spaces$underline");
   }
 
   // TODO(nweiz): Make this handle long lines more gracefully.
   /// Throws a [FormatException] describing that [name] is expected at the
   /// current position in the string.
   void _fail(String name) {
     error("expected $name.", position: this.position, length: 1);
   }
 }