Issue 25251. Fix for extracting string literal part with zero length.

pkg/analysis_server/lib/src/services/refactoring/extract_local.dart

Index: pkg/analysis_server/lib/src/services/refactoring/extract_local.dart
diff --git a/pkg/analysis_server/lib/src/services/refactoring/extract_local.dart b/pkg/analysis_server/lib/src/services/refactoring/extract_local.dart
index f238cd554e5b1e5a398adb4f340dd3d4185aa04b..53ea9848dd2513be7cff57994114670f34fac28c 100644
--- a/pkg/analysis_server/lib/src/services/refactoring/extract_local.dart
+++ b/pkg/analysis_server/lib/src/services/refactoring/extract_local.dart
@@ -281,7 +281,8 @@ class ExtractLocalRefactoringImpl extends RefactoringImpl
     }
     // part of string literal
     if (coveringNode is StringLiteral) {
-      if (selectionRange.offset > coveringNode.offset &&
+      if (selectionRange.length != 0 &&
+          selectionRange.offset > coveringNode.offset &&
           selectionRange.end < coveringNode.end) {
         stringLiteralPart = selectionStr;
         return new RefactoringStatus();