option to enable fasta scanner

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 f2c8e3b001224c418f4f30b0fbcac38c92549003..e416322e76976d4c5327b09fb2f49a60dfebc7f9 100644
--- a/pkg/front_end/lib/src/scanner/scanner.dart
+++ b/pkg/front_end/lib/src/scanner/scanner.dart
@@ -129,7 +129,7 @@ abstract class Scanner {
    * A flag indicating whether the [Scanner] factory method
    * will return a fasta based scanner or an analyzer based scanner.
    */
-  static bool useFasta = false;
+  static bool useFasta = const bool.fromEnvironment("useFastaScanner");
 
   /**
    * The reader used to access the characters in the source.