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 948 matching lines @@
 
   /**
    * If a constructor initializer expression is being serialized, the names of
    * the constructor parameters.  Otherwise `null`.
    */
   final Set<String> constructorParameterNames;
 
   _ConstExprSerializer(this.serializer, this.constructorParameterNames);
 
   @override
+  bool isConstructorParameterName(String name) {
+    return constructorParameterNames?.contains(name) ?? false;
+  }
+
+  @override
   void serializeAnnotation(Annotation annotation) {
     if (annotation.arguments == null) {
       assert(annotation.constructorName == null);
       serialize(annotation.name);
     } else {
       Identifier name = annotation.name;
       Element nameElement = name.staticElement;
       EntityRefBuilder constructor;
       if (nameElement is ConstructorElement && name is PrefixedIdentifier) {
         assert(annotation.constructorName == null);
@@ skipping 31 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 13 matching lines @@
     }
     return new EntityRefBuilder(
         reference: serializer._getElementReferenceId(element));
   }
 
   @override
   EntityRefBuilder serializeType(TypeName typeName) {
     DartType type = typeName != null ? typeName.type : DynamicTypeImpl.instance;
     return serializer.serializeTypeRef(type, null);
   }
-
-  @override
-  bool isConstructorParameterName(String name) {
-    return constructorParameterNames?.contains(name) ?? false;
-  }
 }
 
 /**
  * Instances of this class keep track of intermediate state during
  * serialization of a single library.
  */
 class _LibrarySerializer {
   /**
    * The library to be serialized.
    */
@@ skipping 172 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }