Add bounds check in scope lookup (issue 26633)

pkg/analyzer/lib/src/dart/resolver/scope.dart

Index: pkg/analyzer/lib/src/dart/resolver/scope.dart
diff --git a/pkg/analyzer/lib/src/dart/resolver/scope.dart b/pkg/analyzer/lib/src/dart/resolver/scope.dart
index e90ba8619d8c29e80b562fab5adc395d6b216e2f..04da23d2409956ff40c60c03491573db05c80946 100644
--- a/pkg/analyzer/lib/src/dart/resolver/scope.dart
+++ b/pkg/analyzer/lib/src/dart/resolver/scope.dart
@@ -994,7 +994,7 @@ class PrefixedNamespace implements Namespace {
 
   @override
   Element get(String name) {
-    if (name.startsWith(_prefix)) {
+    if (name.length > _length && name.startsWith(_prefix)) {
       if (name.codeUnitAt(_length) == '.'.codeUnitAt(0)) {
         return _definedNames[name.substring(_length + 1)];
       }