Upgrade to Dart 2.0

[rachel-fenichel]

Problem statement

Dart has upgraded to 2.0, and so has the page we use for testing (dartpad). Some of our generators don't work in 2.0 because there are breaking changes in the language. We need to upgrade, ideally before the release at the start of September.

Expected Behavior

Generated code from the generator tests in Dart can run on DartPad.

Actual Behavior

A variety of type errors.

Steps to Reproduce

Load all tests in the generator test page, and run the generated Dart in DartPad.

Instructions for testing generators: https://developers.google.com/blockly/guides/modify/web/unit-testing#block_generator_tests

[AnmAtAnm]

Assuming there is not a form that is compatible with both 1 and 2, do we need to keep a copy of the Dart 1 generator around for older existing projects? I don't mind deprecating it and taking it out of testing (or test generation without running the resulting code).

Alternatively, we can just point people to the older repo.

[rachel-fenichel]

I think we should just upgrade and tell people to go back in time to get the older generator.

[rachel-fenichel]

Done (thanks Neil)