Fix a small bug in the stress test

pkg/analyzer/lib/src/generated/source.dart

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index 6412de3bf50687ed465ea222c780ee4bbd24481d..bdaf9d96833424545fa41e30264081b85e1f627c 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -265,7 +265,8 @@ class LineInfo {
    */
   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];
   }