Avoid instantiating FileLocations where possible in source_span.

pkg/source_span/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 'package:path/path.dart' as p;
@@ skipping 181 matching lines @@
   /// [start] is lazily generated from this to avoid allocating unnecessary
   /// objects.
   final int _start;
 
   /// The offset of the end of the span.
   ///
   /// [end] is lazily generated from this to avoid allocating unnecessary
   /// objects.
   final int _end;
 
+  Uri get sourceUrl => file.url;
+  int get length => _end - _start;
   FileLocation get start => new FileLocation._(file, _start);
   FileLocation get end => new FileLocation._(file, _end);
-
-  String get text => file.getText(start.offset, end.offset);
+  String get text => file.getText(_start, _end);
 
   FileSpan._(this.file, this._start, this._end) {
     if (_end < _start) {
       throw new ArgumentError('End $_end must come after start $_start.');
     } else if (_end > file.length) {
       throw new RangeError("End $_end must not be greater than the number "
           "of characters in the file, ${file.length}.");
     } else if (_start < 0) {
       throw new RangeError("Start may not be negative, was $_start.");
     }
   }
 
+  int compareTo(SourceSpan other) {
+    if (other is! FileSpan) return super.compareTo(other);
+
+    FileSpan otherFile = other;
+    var result = _start.compareTo(otherFile._start);
+    return result == 0 ? _end.compareTo(otherFile._end) : result;
+  }
+
   SourceSpan union(SourceSpan other) {
     if (other is! FileSpan) return super.union(other);
 
     var span = expand(other);
-    var beginSpan = span.start == this.start ? this : other;
-    var endSpan = span.end == this.end ? this : other;
+    var beginSpan = span._start == _start ? this : other;
+    var endSpan = span._end == _end ? this : other;
 
-    if (beginSpan.end.compareTo(endSpan.start) < 0) {
+    if (beginSpan._end < endSpan._start) {
       throw new ArgumentError("Spans $this and $other are disjoint.");
     }
 
     return span;
   }
 
+  bool operator ==(other) {
+    if (other is! FileSpan) return super == other;
+    return _start == other._start && _end == other._end &&
+        sourceUrl == other.sourceUrl;
+  }
+
+  int get hashCode => _start.hashCode + 5 * _end.hashCode +
+      7 * sourceUrl.hashCode;
+
   /// Returns a new span that covers both [this] and [other].
   ///
   /// Unlike [union], [other] may be disjoint from [this]. If it is, the text
   /// between the two will be covered by the returned span.
   FileSpan expand(FileSpan other) {
     if (sourceUrl != other.sourceUrl) {
       throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
           " \"${other.sourceUrl}\" don't match.");
     }
 
     var start = math.min(this._start, other._start);
     var end = math.max(this._end, other._end);
-    return new FileSpan._(file, start, end);    
+    return new FileSpan._(file, start, end);
   }
 
   String message(String message, {color}) {
     if (color == true) color = colors.RED;
     if (color == false) color = null;
 
     var line = start.line;
     var column = start.column;
 
     var buffer = new StringBuffer();
@@ skipping 19 matching lines @@
     }
     if (!textLine.endsWith('\n')) buffer.write('\n');
 
     buffer.write(' ' * column);
     if (color != null) buffer.write(color);
     buffer.write('^' * math.max(toColumn - column, 1));
     if (color != null) buffer.write(colors.NONE);
     return buffer.toString();
   }
 }