Fix recently introduced strong mode errors

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/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/type.dart' show DartType;
@@ skipping 1076 matching lines @@
                     localClosureIndexMap, expr, constructorParameterNames);
               }))
           .toList();
     }
     executables.add(b);
   }
 
   @override
   UnlinkedParamBuilder visitDefaultFormalParameter(
       DefaultFormalParameter node) {
-    UnlinkedParamBuilder b = node.parameter.accept(this);
+    UnlinkedParamBuilder b =
+        node.parameter.accept(this) as UnlinkedParamBuilder;
     b.initializer = serializeInitializerFunction(node.defaultValue, true);
     if (node.defaultValue != null) {
       b.defaultValueCode = node.defaultValue.toSource();
     }
     b.codeRange = serializeCodeRange(node);
     return b;
   }
 
   @override
   void visitEnumDeclaration(EnumDeclaration node) {
@@ skipping 269 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;
 }