Improve the speed and memory efficiency of csslib parsing.

pkg/csslib/lib/src/token.dart

 // Copyright (c) 2012, 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.
 
 part of csslib.parser;
 
 /**
  * A single token in the Dart language.
  */
 class Token {
   /** A member of [TokenKind] specifying what kind of token this is. */
   final int kind;
 
   /** The location where this token was parsed from. */
-  final SourceSpan span;
+  final FileSpan span;
 
   /** The start offset of this token. */
   int get start => span.start.offset;
 
   /** The end offset of this token. */
   int get end => span.end.offset;
 
   /** Returns the source text corresponding to this [Token]. */
   String get text => span.text;
 
@@ skipping 10 matching lines @@
       return '$kindText($actualText)';
     } else {
       return kindText;
     }
   }
 }
 
 /** A token containing a parsed literal value. */
 class LiteralToken extends Token {
   var value;
-  LiteralToken(int kind, SourceSpan span, this.value) : super(kind, span);
+  LiteralToken(int kind, FileSpan span, this.value) : super(kind, span);
 }
 
 /** A token containing error information. */
 class ErrorToken extends Token {
   String message;
-  ErrorToken(int kind, SourceSpan span, this.message) : super(kind, span);
+  ErrorToken(int kind, FileSpan span, this.message) : super(kind, span);
 }
 
 /**
  * CSS ident-token.
  *
  * See <http://dev.w3.org/csswg/css-syntax/#typedef-ident-token> and
  * <http://dev.w3.org/csswg/css-syntax/#ident-token-diagram>.
  */
 class IdentifierToken extends Token {
   final String text;
 
-  IdentifierToken(this.text, int kind, SourceSpan span)
+  IdentifierToken(this.text, int kind, FileSpan span)
       : super(kind, span);
 }