chore(dart/transform): Create targets for serving transformed Dart code

- Allow pub (build|serve) to specify mode
- Update pubbuild.js & pubserve.js to allow the caller to provide a `mode` value.
- Update settings to allow the di benchmark to be transformed to run statically.

tools/build/pubbuild.js

@@ -17,7 +17,9 @@ module.exports = function(gulp, plugins, config) {
       }
       var folder = path.resolve(path.join(webFolders.shift(), '..'));
       var destFolder = path.resolve(path.join(config.dest, path.basename(folder)));
-      return util.processToPromise(spawn(config.command, ['build', '-o', destFolder], {
+      var pubMode = config.mode || 'release';
+      var pubArgs = ['build', '--mode', pubMode, '-o', destFolder];
+      return util.processToPromise(spawn(config.command, pubArgs, {
         stdio: 'inherit',
         cwd: folder
       })).then(function() {