Tighten the span around index operators.

lib/src/chunk.dart

Index: lib/src/chunk.dart
diff --git a/lib/src/chunk.dart b/lib/src/chunk.dart
index da6d7bbc011ac9e7d82055ec4d43b76b66c48b52..0ac36bd07f1bcbe424f636e0b359b4101f580ebb 100644
--- a/lib/src/chunk.dart
+++ b/lib/src/chunk.dart
@@ -329,6 +329,9 @@ class Cost {
   /// Splitting on the "." in a named constructor.
   static const constructorName = 3;
 
+  /// Splitting a `[...]` index operator.
+  static const index = 3;
+
   /// Splitting before a type argument or type parameter.
   static const typeArgument = 4;
 }