add tool to get breakdown of deferred libraries by size

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,
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   * [`library_size_split`][lib_split]: a tool that shows how much code was
     attributed to each library. This tool is configurable so it can group data
     in many ways (e.g. to tally together all libraries that belong to a package,
     or all libraries that match certain name pattern).
 
   * [`deferred_library_check`][deferred_lib]: a tool that verifies that code
     was split into deferred parts as expected. This tool takes a specification
     of the expected layout of code into deferred parts, and checks that the
     output from `dart2js` meets the specification.
+    
+  * [`deferred_library_size`][deferred_size]: a tool that gives a breakdown of
+    the sizes of the deferred parts of the program. This can show how much of
+    your total code size can be loaded deferred.
 
   * [`function_size_analysis`][function_analysis]: a tool that shows how much
     code was attributed to each function. This tool also uses dependency
     information to compute dominance and reachability data. This information can
     sometimes help determine how much savings could come if the function was not
     included in the program.
 
   * [`coverage_log_server`][coverage] and [`live_code_size_analysis`][live]:
     dart2js has an experimental feature to gather coverage data of your
     application. The `coverage_log_server` can record this data, and
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 contained in that part, a list of package names that are expected to be in
 another part, or both. For instance, in the example YAML above the part named
 "baz" is expected to contain the packages "baz" and "quux" and exclude the
 package "zardoz".
 
 The names for parts given in the specification YAML file (besides "main")
 are the same as the name given to the deferred import in the dart file. For
 instance, if you have `import 'package:foo/bar.dart' deferred as baz;` in your
 dart file, then the corresponding name in the specification file is 'baz'.
 
+### Deferred library size tool
+
+This tool gives a breakdown of all of the deferred code in the program by size.
+It can show how much of the total code size is deferred. It can be run as
+follows:
+
+```bash
+pub global activate dart2js_info # only needed once
+dart2js_info_deferred_library_size out.js.info.json
+```
+
+The tool will output a table listing all of the deferred imports in the program
+as well as the "main" chunk, which is not deferred. The output looks like:
+
+```
+Size by library
+------------------------------------------------
+main                                    12345678
+foo                                      7654321
+bar                                      1234567
+------------------------------------------------
+Main chunk size                         12345678
+Deferred code size                       8888888
+Percent of code deferred                  41.86%
+```
+
 ### Function size analysis tool
 
 This command-line tool presents how much each function contributes to the total
 code of your application.  We use dependency information to compute dominance
 and reachability data as well.
 
 When you run:
 ```bash
 pub global activate dart2js_info # only needed once
 dart2js_info_function_size_analysis out.js.info.json
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 ## Code location, features and bugs
 
 This package is developed in [github][repo].  Please file feature requests and
 bugs at the [issue tracker][tracker].
 
 [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
 [deferred_lib]: https://github.com/dart-lang/dart2js_info/blob/master/bin/deferred_library_check.dart
+[deferred_size]: https://github.com/dart-lang/dart2js_info/blob/master/bin/deferred_library_size.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
 [AllInfo]: http://dart-lang.github.io/dart2js_info/doc/api/dart2js_info.info/AllInfo-class.html