Improve support of Dart SDK 2.0

[cedx]

The latest release of the Dart SDK (i.e. version 2.0.0-dev.69 at the time of writing) let us verify that a package and its dependencies are ready for the upcoming stable Dart 2 SDK (thanks to the _PUB_TEST_SDK_VERSION environment variable).

I've a project that depends on the platform package (v2.1.2), and there is an issue:

$ _PUB_TEST_SDK_VERSION=2.0.0 pub upgrade
Resolving dependencies... (9.3s)
The current Dart SDK version is 2.0.0.

Because my_project depends on platform >=1.0.1 which requires SDK version >=1.19.0 <2.0.0-dev.infinity, version solving failed.

[tvolkert]

This should be fixed now