Issue 22409. Restore recognizing string concatenation in const expressions.

pkg/analyzer/test/generated/non_error_resolver_test.dart

Index: pkg/analyzer/test/generated/non_error_resolver_test.dart
diff --git a/pkg/analyzer/test/generated/non_error_resolver_test.dart b/pkg/analyzer/test/generated/non_error_resolver_test.dart
index 1761c81ad4c56c564a0633587bd174c70b7d1eb7..034544f3208361f852344ff35e8c87a9732a8dbf 100644
--- a/pkg/analyzer/test/generated/non_error_resolver_test.dart
+++ b/pkg/analyzer/test/generated/non_error_resolver_test.dart
@@ -1167,6 +1167,16 @@ class B {
     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 {