Fix incremental scanner's computation of scanEnd.

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

Index: pkg/analyzer/lib/src/generated/incremental_scanner.dart
diff --git a/pkg/analyzer/lib/src/generated/incremental_scanner.dart b/pkg/analyzer/lib/src/generated/incremental_scanner.dart
index f7e72efeeb80ecf334d9c94de0c6d04834b5ea7d..ac91b9634a3bbedb8c3bdeb41a75822f1dd668cc 100644
--- a/pkg/analyzer/lib/src/generated/incremental_scanner.dart
+++ b/pkg/analyzer/lib/src/generated/incremental_scanner.dart
@@ -136,8 +136,8 @@ class IncrementalScanner {
     // If the index is within an existing token, then we need to start at the
     // beginning of the token.
     //
-    int scanStart = math.max(oldFirst.previous.end, 0);
-    int scanEnd = oldLast.end + delta;
+    int scanStart = math.max(oldLeftToken.end, 0);
+    int scanEnd = oldRightToken.offset + delta;
     //
     // Rescan the characters that need to be rescanned.
     //