Rewrite APISummarizer in Dart

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](https://github.com/dart-lang/native/pull/1141), [i/o issues](https://github.com/dart-lang/native/issues/655), 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.

Start by checking whether JNIgen has reached 1.0, then locate APISummarizer and the Java tool it would replace. Confirm how JNIgen currently consumes the JSON output; done means APISummarizer is implemented in Dart, directly returns the Dart classes used by JNIgen, and avoids the stated encoding, I/O, parsing, and dependency-cycle concerns.

Written by the indexing model from the issue text.