Allow literal symbols as constant expressions.

pkg/front_end/lib/src/fasta/parser/identifier_context.dart

Index: pkg/front_end/lib/src/fasta/parser/identifier_context.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/identifier_context.dart b/pkg/front_end/lib/src/fasta/parser/identifier_context.dart
index 1f3dad56250c7a77aab4d57068306c181ef60fcf..29152efd84dcf56bf1278b91ce866b4f81fc2639 100644
--- a/pkg/front_end/lib/src/fasta/parser/identifier_context.dart
+++ b/pkg/front_end/lib/src/fasta/parser/identifier_context.dart
@@ -291,7 +291,8 @@ class IdentifierContext {
             // Generally, declarations are legal in constant expressions.  A
             // continuation doesn't affect constant expressions: if what it's
             // continuing is a problem, it has already been reported.
-            allowedInConstantExpression ?? (inDeclaration || isContinuation);
+            allowedInConstantExpression ??
+                (inDeclaration || isContinuation || inSymbol);
 
   String toString() => _name;
 }