add a JsonInfoCodec class

README.md

 # Dart2js Info
 
 This package contains libraries and tools you can use to process `.info.json`
 files, which are produced when running dart2js with `--dump-info`.
 
 The `.info.json` files contain data about each element included in
 the output of your program. The data includes information such as:
 
   * the size that each function adds to the `.dart.js` output,
   * dependencies between functions,
@@ skipping 19 matching lines @@
 add more data to the `.info.json` files, and include better tools to help
 understand the results of type inference.
 
 This package is still in flux and we might make breaking changes at any time.
 Our current goal is not to provide a stable API, we mainly want to expose the
 functionality and iterate on it.  We recommend that you pin a specific version
 of this package and update when needed.
 
 ## Info API
 
-[AllInfo][AllInfo] exposes a Dart representation of the `.info.json` files.
-You can parse the information using `AllInfo.fromJson`. For example:
+[AllInfo][AllInfo] exposes a Dart representation of all of the collected
+information. You can decode an `AllInfo` object from the JSON form produced by
+the `dart2js` `--dump-info` option using the `AllInfoJsonCodec`. For example:
 
 ```dart
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:dart2js_info/info.dart';
 
 main(args) {
   var infoPath = args[0];
   var json = JSON.decode(new File(infoPath).readAsStringSync());
-  var info = new AllInfo.fromJson(json);
+  var info = new AllInfoJsonCodec().decode(json);
   ...
 }
 ```
 
 ## Available tools
 
 The following tools are a available today:
 
   * [`code_deps.dart`][code_deps]: simple tool that can answer queries about the
     dependency between functions and fields in your program. Currently it only
@@ skipping 223 matching lines @@
 
 [repo]: https://github.com/dart-lang/dart2js_info/
 [tracker]: https://github.com/dart-lang/dart2js_info/issues
 [code_deps]: https://github.com/dart-lang/dart2js_info/blob/master/bin/code_deps.dart
 [lib_split]: https://github.com/dart-lang/dart2js_info/blob/master/bin/library_size_split.dart
 [coverage]: https://github.com/dart-lang/dart2js_info/blob/master/bin/coverage_log_server.dart
 [live]: https://github.com/dart-lang/dart2js_info/blob/master/bin/live_code_size_analysis.dart
 [function_analysis]: https://github.com/dart-lang/dart2js_info/blob/master/bin/function_size_analysis.dart
 [verify_deps]: https://github.com/dart-lang/dart2js_info/blob/master/bin/verify_deps.dart
 [AllInfo]: http://dart-lang.github.io/dart2js_info/doc/api/dart2js_info.info/AllInfo-class.html