Make ts2dart broccoli plugin incremental

the existing [ts2dart plugin](https://github.com/angular/angular/blob/ab28676d0260772b477b6382cfa4f367b67fee60/tools/broccoli/broccoli-ts2dart.ts) needs to be converted into [DiffingBroccoliPlugin](https://github.com/angular/angular/blob/ab28676d0260772b477b6382cfa4f367b67fee60/tools/broccoli/diffing-broccoli-plugin.ts) to make it incremental

Depends on #1719
