Allow literal symbols as constant expressions.

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

 // Copyright (c) 2017, 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.
 
 /// Information about the parser state which is passed to the listener at the
 /// time an identifier is encountered.
 ///
 /// This can be used by the listener to determine the context in which the
 /// identifier appears; that in turn can help the listener decide how to resolve
 /// the identifier (if the listener is doing resolution).
@@ skipping 273 matching lines @@
       this.inLibraryOrPartOfDeclaration: false,
       this.inSymbol: false,
       this.isContinuation: false,
       this.isScopeReference: false,
       this.isBuiltInIdentifierAllowed: true,
       bool allowedInConstantExpression})
       : this.allowedInConstantExpression =
             // 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;
 }