[jnigen] JAR handling improvements (https://github.com/dart-lang/jnigen/issues/220)

* Move summarizer invocation into summary.dart for better testability.
* Support reading source JARs (https://github.com/dart-lang/jnigen/issues/208)
* Fix an issue where the summarizer failed when providing only classes and no source (https://github.com/dart-lang/jnigen/issues/147)
15 files changed
tree: 3afba8e0c6576970ca7959ea696d91bb28fd1da4
  1. .github/
  2. pkgs/