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

pkg/front_end/lib/src/scanner/syntactic_entity.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.
-
-/**
- * Interface representing a syntactic entity (either a token or an AST node)
- * which has a location and extent in the source file.
- */
-abstract class SyntacticEntity {
-  /**
-   * Return the offset from the beginning of the file to the character after the
-   * last character of the syntactic entity.
-   */
-  int get end;
-
-  /**
-   * Return the number of characters in the syntactic entity's source range.
-   */
-  int get length;
-
-  /**
-   * Return the offset from the beginning of the file to the first character in
-   * the syntactic entity.
-   */
-  int get offset;
-}