Fix a performance bug with FileLocation.

lib/src/location.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.location;
 
 import 'span.dart';
 
 // A class that describes a single location within a source file.
 class SourceLocation implements Comparable<SourceLocation> {
@@ skipping 25 matching lines @@
   ///
   /// [line] and [column] default to assuming the source is a single line. This
   /// means that [line] defaults to 0 and [column] defaults to [offset].
   ///
   /// [sourceUrl] may be either a [String], a [Uri], or `null`.
   SourceLocation(int offset, {sourceUrl, int line, int column})
       : sourceUrl = sourceUrl is String ? Uri.parse(sourceUrl) : sourceUrl,
         offset = offset,
         line = line == null ? 0 : line,
         column = column == null ? offset : column {
-    if (this.offset < 0) {
-      throw new RangeError("Offset may not be negative, was ${this.offset}.");
-    } else if (this.line < 0) {
-      throw new RangeError("Line may not be negative, was ${this.line}.");
-    } else if (this.column < 0) {
-      throw new RangeError("Column may not be negative, was ${this.column}.");
+    if (offset < 0) {
+      throw new RangeError("Offset may not be negative, was $offset.");
+    } else if (line != null && line < 0) {
+      throw new RangeError("Line may not be negative, was $line.");
+    } else if (column != null && column < 0) {
+      throw new RangeError("Column may not be negative, was $column.");
     }
   }
 
   /// Returns the distance in characters between [this] and [other].
   ///
   /// This always returns a non-negative value.
   int distance(SourceLocation other) {
     if (sourceUrl != other.sourceUrl) {
       throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
           "\"${other.sourceUrl}\" don't match.");
@@ skipping 16 matching lines @@
   }
 
   bool operator ==(other) =>
       other is SourceLocation && sourceUrl == other.sourceUrl &&
       offset == other.offset;
 
   int get hashCode => sourceUrl.hashCode + offset;
 
   String toString() => '<$runtimeType: $offset $toolString>';
 }