Fix a small bug in the stress test

pkg/analyzer/lib/src/generated/source.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 analyzer.src.generated.source;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 247 matching lines @@
 
     return new LineInfo_Location(min + 1, offset - _lineStarts[min] + 1);
   }
 
   /**
    * Return the offset of the first character on the line with the given
    * [lineNumber].
    */
   int getOffsetOfLine(int lineNumber) {
     if (lineNumber < 0 || lineNumber >= lineCount) {
-      throw new ArgumentError('Invalid line number: $lineNumber');
+      throw new ArgumentError(
+          'Invalid line number: $lineNumber; must be between 0 and ${lineCount - 1}');
     }
     return _lineStarts[lineNumber];
   }
 }
 
 /**
  * Instances of the class `Location` represent the location of a character as a line and
  * column pair.
  */
 class LineInfo_Location {
@@ skipping 654 matching lines @@
   Source resolveAbsolute(Uri uri, [Uri actualUri]);
 
   /**
    * Return an absolute URI that represents the given [source], or `null` if a
    * valid URI cannot be computed.
    *
    * The computation should be based solely on [source.fullName].
    */
   Uri restoreAbsolute(Source source) => null;
 }