Add UnlinkedConst.isValid and set it during summarizing.

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

 // Copyright (c) 2015, 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.elements;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/type.dart';
@@ skipping 420 matching lines @@
       b.hides = combinator.hiddenNames;
     }
     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 documentation from the given [element], creating an
    * [UnlinkedDocumentationComment].
    *
    * If [element] has no documentation, `null` is returned.
    */
   UnlinkedDocumentationCommentBuilder serializeDocumentation(Element element) {
     if (element.documentationComment == null) {
@@ skipping 485 matching lines @@
     }
   }
 
   EntityRefBuilder serializeIdentifier(Identifier identifier) {
     Element element = identifier.staticElement;
     assert(element != null);
     // The only supported instance property accessor - `length`.
     if (identifier is PrefixedIdentifier &&
         element is PropertyAccessorElement &&
         !element.isStatic) {
-      assert(element.name == 'length');
+      if (element.name != 'length') {
+        throw new StateError('Only "length" property is allowed in constants.');
+      }
       Element prefixElement = identifier.prefix.staticElement;
       int prefixRef = serializer._getElementReferenceId(prefixElement);
       int lengthRef = serializer._getLengthPropertyReference(prefixRef);
       return new EntityRefBuilder(reference: lengthRef);
     }
     return new EntityRefBuilder(
         reference: serializer._getElementReferenceId(element));
   }
 
   @override
@@ skipping 205 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }