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

scripts/travis/build.sh

@@ -8,10 +8,6 @@ SCRIPT_DIR=$(dirname $0)
 cd $SCRIPT_DIR/../..
 source ./scripts/env.sh
 
-# For some reason, this task fails on Travis when run as a part of the `gulp build`.
-# It runs `pub get` which fails to read the `pubspec.yml` (created earlier by the task).
-./node_modules/.bin/gulp modules/build.dart/pubspec
-
 ./node_modules/.bin/gulp build
 
 pub install