refactor(build): simplify and modularize

simplify:
- use same html file for dart and JS
- build benchmarks automatically when doing `gulp build`
- centralize configuration

modularize:
- move all build tasks into separate node.js modules under
  `tools/build`.

changes:
- the `build` folder is now the `dist` folder

Closes #284

modules/benchmarks_external/src/compiler/main.es6

@@ -0,0 +1 @@
+export {main} from './compiler_benchmark_ng13';