enable fasta scanner in analyzer by default

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

Index: pkg/front_end/lib/src/scanner/scanner.dart
diff --git a/pkg/front_end/lib/src/scanner/scanner.dart b/pkg/front_end/lib/src/scanner/scanner.dart
index e532eda02ce59eacc7bc941d6c9c9d187bc3b5f1..d20ba0baea2a316ae8f7f2695db1125830bebf1d 100644
--- a/pkg/front_end/lib/src/scanner/scanner.dart
+++ b/pkg/front_end/lib/src/scanner/scanner.dart
@@ -129,7 +129,8 @@ 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.