enable fasta scanner in analyzer by default

pkg/front_end/lib/src/scanner/scanner.dart

 // Copyright (c) 2016, 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:charcode/ascii.dart';
 import 'package:front_end/src/scanner/errors.dart';
 import 'package:front_end/src/scanner/reader.dart';
 import 'package:front_end/src/scanner/string_utilities.dart';
 import 'package:front_end/src/scanner/token.dart';
 
@@ skipping 111 matching lines @@
  * determine whether source of the form "<<" should be scanned as a single
  * left-shift operator or as two left angle brackets. This scanner does not have
  * any context, so it always resolves such conflicts by scanning the longest
  * possible token.
  */
 abstract class Scanner {
   /**
    * A flag indicating whether the analyzer [Scanner] factory method
    * will return a fasta based scanner or an analyzer based scanner.
    */
-  static bool useFasta = const bool.fromEnvironment("useFastaScanner");
+  static bool useFasta =
+      const bool.fromEnvironment("useFastaScanner", defaultValue: true);
 
   /**
    * The reader used to access the characters in the source.
    */
   final CharacterReader _reader;
 
   /**
    * The flag specifying whether documentation comments should be parsed.
    */
   bool _preserveComments = true;
@@ skipping 1191 matching lines @@
   }
 
   /**
    * Checks if [value] is a single-line or multi-line comment.
    */
   static bool _isDocumentationComment(String value) {
     return StringUtilities.startsWith3(value, 0, $slash, $slash, $slash) ||
         StringUtilities.startsWith3(value, 0, $slash, $asterisk, $asterisk);
   }
 }