All strong mode cleaning of dart2js.

pkg/compiler/lib/src/serialization/json_serializer.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 dart2js.serialization.json;
 
 import 'dart:convert';
 
 import 'keys.dart';
 import 'serialization.dart';
@@ skipping 90 matching lines @@
   String visitString(StringValue value, arg) => value.value;
 
   @override
   visitType(TypeValue value, arg) => visit(value.id);
 
   @override
   visitUri(UriValue value, arg) => '${value.value}';
 }
 
 /// [ValueVisitor] that generates a verbose JSON-like output.
-class PrettyPrintEncoder implements ValueVisitor {
+class PrettyPrintEncoder implements ValueVisitor<dynamic, String> {
   StringBuffer buffer;
 
   String toText(Value value) {
     buffer = new StringBuffer();
     visit(value, '');
     String text = buffer.toString();
     buffer = null;
     return text;
   }
 
@@ skipping 100 matching lines @@
   @override
   void visitType(TypeValue value, String indentation) {
     buffer.write('Type(${value.id}):${value.type}');
   }
 
   @override
   void visitUri(UriValue value, String indentation) {
     buffer.write('Uri(${value.value})');
   }
 }