Rewrite APISummarizer in Dart

[HosseinYousefi]

Once JNIgen hits 1.0, we can rewrite APISummarize in Dart itself using JNIgen! This way we don't have to deal with encoding issues, i/o issues, and having to maintain a Java tool.

APISummarizer can be somewhat separated and instead of outputting JSON and parsing it again, it directly returns Dart classes used by JNIgen. Since JNIgen is a dev dependency, there won't be any dependency cycles between the two tools.