Add UnlinkedConst.isValid and set it during summarizing.

pkg/analyzer/lib/src/summary/summarize_ast.dart

 // Copyright (c) 2016, 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 serialization.summarize_ast;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
@@ skipping 374 matching lines @@
     b.variables = variables;
     b.publicNamespace = computePublicNamespace(compilationUnit);
     return b;
   }
 
   /**
    * Serialize the given [expression], creating an [UnlinkedConstBuilder].
    */
   UnlinkedConstBuilder serializeConstExpr(Expression expression) {
     _ConstExprSerializer serializer = new _ConstExprSerializer(this);
-    serializer.serialize(expression);
-    return serializer.toBuilder();
+    return serializer.serialize(expression);
   }
 
   /**
    * Serialize a [Comment] node into an [UnlinkedDocumentationComment] object.
    */
   UnlinkedDocumentationCommentBuilder serializeDocumentation(
       Comment documentationComment) {
     if (documentationComment == null) {
       return null;
     }
@@ skipping 484 matching lines @@
 /**
  * A [_TypeParameterScope] is a [_Scope] which defines [_ScopedTypeParameter]s.
  */
 class _TypeParameterScope extends _Scope {
   /**
    * Get the number of [_ScopedTypeParameter]s defined in this
    * [_TypeParameterScope].
    */
   int get length => _definedNames.length;
 }