[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)