Fix field constants verifying with CFE.

TBR

R=brianwilkerson@google.com

Change-Id: I7960249e95c48cb51d8ca593df93381398e94d8a
Reviewed-on: https://dart-review.googlesource.com/68400
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart

@@ -801,6 +801,10 @@ class LibraryAnalyzer {
             fieldList.type?.accept(applier);
             for (var field in fields.reversed) {
               field.initializer?.accept(applier);
+              if (element is ConstVariableElement) {
+                (element as ConstVariableElement).constantInitializer =
+                    field.initializer;
+              }
             }
             applier.applyToAnnotations(member);
           } else if (member is MethodDeclaration) {