Add server utilities for 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 1a4751678597008be0e04437eaa118bd5e69fd35..5327c984988ba4c498347c3aa6829290bf119987 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -209,6 +209,17 @@ class LineInfo {
   }
 
   /**
+   * Return the offset of the first character on the line with the given
+   * [lineNumber].
+   */
+  int getLineOffset(int lineNumber) {
+    if (lineNumber < 0 || lineNumber >= _lineStarts.length) {
+      throw new ArgumentError('Invalid line number: $lineNumber');
+    }
+    return _lineStarts[lineNumber];
+  }
+
+  /**
    * Return the location information for the character at the given offset.
    *
    * @param offset the offset of the character for which location information is to be returned