Add bounds check in scope lookup (issue 26633)

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.dart.resolver.scope;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 976 matching lines @@
   Map<String, Element> get definedNames {
     Map<String, Element> definedNames = <String, Element>{};
     _definedNames.forEach((String name, Element element) {
       definedNames["$_prefix.$name"] = element;
     });
     return definedNames;
   }
 
   @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)];
       }
     }
     return null;
   }
 
   @override
   Element getPrefixed(String prefix, String name) {
     if (prefix == _prefix) {
@@ skipping 201 matching lines @@
 
   /**
    * Define the type parameters declared by the [classElement].
    */
   void _defineTypeParameters(ClassElement classElement) {
     for (TypeParameterElement typeParameter in classElement.typeParameters) {
       define(typeParameter);
     }
   }
 }