Issue 22409. Restore recognizing string concatenation in const expressions.

pkg/analyzer/test/generated/non_error_resolver_test.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 engine.non_error_resolver_test;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/parser.dart' show ParserErrorCode;
@@ skipping 1149 matching lines @@
   const B.c4(String p) : v = p != 0.0;
   const B.c5(String p) : v = p != '';
   const B.n1(num p) : v = p != null;
   const B.n2(num p) : v = null != p;
 }''');
     resolve(source);
     assertNoErrors(source);
     verify([source]);
   }
 
+  void test_constEvelTypeNum_String() {
+    Source source = addSource(r'''
+const String A = 'a';
+const String B = A + 'b';
+''');
+    resolve(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
   void test_constMapKeyExpressionTypeImplementsEquals_abstract() {
     Source source = addSource(r'''
 abstract class B {
   final id;
   const B(this.id);
   String toString() => 'C($id)';
   /** Equality is identity equality, the id isn't used. */
   bool operator==(Object other);
   }
 
@@ skipping 4176 matching lines @@
     resolve(source);
     assertNoErrors(source);
     verify([source]);
     reset();
   }
 
   void _check_wrongNumberOfParametersForOperator1(String name) {
     _check_wrongNumberOfParametersForOperator(name, "a");
   }
 }