Add SourceLocationMixin and SourceLocationBase.

lib/src/file.dart

 // Copyright (c) 2014, 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.
 
 library source_span.file;
 
 import 'dart:math' as math;
 import 'dart:typed_data';
 
 import 'location.dart';
+import 'location_mixin.dart';
 import 'span.dart';
 import 'span_mixin.dart';
 import 'span_with_context.dart';
 
 // Constants to determine end-of-lines.
 const int _LF = 10;
 const int _CR = 13;
 
 /// A class representing a source file.
 ///
@@ skipping 184 matching lines @@
   String getText(int start, [int end]) =>
       new String.fromCharCodes(_decodedChars.sublist(start, end));
 }
 
 /// A [SourceLocation] within a [SourceFile].
 ///
 /// Unlike the base [SourceLocation], [FileLocation] lazily computes its line
 /// and column values based on its offset and the contents of [file].
 ///
 /// A [FileLocation] can be created using [SourceFile.location].
-class FileLocation extends SourceLocation {
+class FileLocation extends SourceLocationMixin implements SourceLocation {
   /// The [file] that [this] belongs to.
   final SourceFile file;
 
+  final int offset;
   Uri get sourceUrl => file.url;
   int get line => file.getLine(offset);
   int get column => file.getColumn(offset);
 
-  FileLocation._(this.file, int offset)
-      : super(offset) {
-    if (offset > file.length) {
+  FileLocation._(this.file, this.offset) {
+    if (offset < 0) {
+      throw new RangeError("Offset may not be negative, was $offset.");
+    } else if (offset > file.length) {
       throw new RangeError("Offset $offset must not be greater than the number "
           "of characters in the file, ${file.length}.");
     }
   }
 
   FileSpan pointSpan() => new _FileSpan(file, offset, offset);
 }
 
 /// A [SourceSpan] within a [SourceFile].
 ///
@@ skipping 107 matching lines @@
       var start = math.min(this._start, other._start);
       var end = math.max(this._end, other._end);
       return new _FileSpan(file, start, end);
     } else {
       var start = math.min(this._start, other.start.offset);
       var end = math.max(this._end, other.end.offset);
       return new _FileSpan(file, start, end);
     }
   }
 }