[pigeon] Port over several test suites from run_tests.sh to run_tests…

….dart (flutter#532)

* Port over several test suites from run_tests.sh to run_tests.dart

This ports over
 - dart_unittests
 - flutter_unittests
 - mock_handler_tests

* [pigeon] Update version and changelog

[pigeon] fixed style issue with implied return null (flutter#544)

[flutter_markdown] Ignore leading space for p and li (flutter#517)

[pigeon] fixed null support for java enums (flutter#528)

[pigeon] Made using enums in type arguments an error, added workaround (

flutter#527)

[metrics_center] Filter out host_name, load_avg and caches keys from …

…context for MetricPoint generation (flutter#515)

[pigeon] fixed bug where the codec didn't contain all types when "Obj…

…ect" is used. (flutter#510)

* [pigeon] fixed bug where the codec didn't contain all types when "Object" is used.

* stuarts feedback

[cupertino_icons] Update README to link to API docs (flutter#508)

[pigeon] fixed cast error that can happen with FlutterApi and primiti…

…ve lists (flutter#506)

[cross_file] Use Blobs to store files on the web. (flutter#494)