Fix exception in code completion (issue 23439)

pkg/analysis_server/lib/src/services/completion/local_declaration_visitor.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 services.completion.dart.local.declaration.visitor;
 
 import 'package:analysis_server/src/services/completion/suggestion_builder.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 
@@ skipping 142 matching lines @@
     SimpleIdentifier id;
     TypeName type;
     DeclaredIdentifier loopVar = node.loopVariable;
     if (loopVar != null) {
       id = loopVar.identifier;
       type = loopVar.type;
     } else {
       id = node.identifier;
       type = null;
     }
-    declaredLocalVar(id, type);
+    if (id != null) {
+      // If there is no loop variable, don't declare it.
+      declaredLocalVar(id, type);
+    }
     visitNode(node);
   }
 
   @override
   void visitForStatement(ForStatement node) {
     VariableDeclarationList varList = node.variables;
     if (varList != null) {
       varList.variables.forEach((VariableDeclaration varDecl) {
         declaredLocalVar(varDecl.name, varList.type);
       });
@@ skipping 86 matching lines @@
       });
     }
   }
 }
 
 /**
  * Internal exception used to indicate that [LocalDeclarationVisitor]
  * should stop visiting.
  */
 class _LocalDeclarationVisitorFinished {}