Fix more strong mode errors in analyzer

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 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 486 matching lines @@
   }
 
   /**
    * Serialize annotations from the given [element].  If [element] has no
    * annotations, the empty list is returned.
    */
   List<UnlinkedConstBuilder> serializeAnnotations(Element element) {
     if (element.metadata.isEmpty) {
       return const <UnlinkedConstBuilder>[];
     }
-    return element.metadata.map((ElementAnnotationImpl a) {
+    return element.metadata.map((ElementAnnotation a) {
       _ConstExprSerializer serializer =
           new _ConstExprSerializer(this, element, null);
-      serializer.serializeAnnotation(a.annotationAst);
+      serializer
+          .serializeAnnotation((a as ElementAnnotationImpl).annotationAst);
       return serializer.toBuilder();
     }).toList();
   }
 
   /**
    * Return the index of the entry in the references table
    * ([LinkedLibrary.references]) used for the "bottom" type.  A new entry is
    * added to the table if necessary to satisfy the request.
    */
   int serializeBottomReference() {
@@ skipping 1099 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }