Create pkg/front_end/src/base and begin populating with general use classes.

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

Index: pkg/front_end/lib/src/scanner/token.dart
diff --git a/pkg/front_end/lib/src/scanner/token.dart b/pkg/front_end/lib/src/scanner/token.dart
index 10d49247d4a0ac3f97ca12fd6a8f1fb9aa55f230..95c1005922d9e8f245c236a8b381ca2a3e0d6934 100644
--- a/pkg/front_end/lib/src/scanner/token.dart
+++ b/pkg/front_end/lib/src/scanner/token.dart
@@ -6,11 +6,10 @@
  * Defines the tokens that are produced by the scanner, used by the parser, and
  * referenced from the [AST structure](ast.dart).
  */
-
 import 'dart:collection';
 
+import 'package:front_end/src/base/syntactic_entity.dart';
 import 'package:front_end/src/scanner/string_utilities.dart';
-import 'package:front_end/src/scanner/syntactic_entity.dart';
 
 /**
  * The opening half of a grouping pair of tokens. This is used for curly