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/pubserve.js

@@ -3,7 +3,9 @@ var spawn = require('child_process').spawn;
 
 module.exports = function(gulp, plugins, config, module) {
   return function() {
-    return util.streamToPromise(spawn(config.command, ['serve'], {
+    var pubMode = config.mode || 'debug';
+    var pubArgs = ['serve', '--mode', pubMode];
+    return util.streamToPromise(spawn(config.command, pubArgs, {
       cwd: config.path, stdio: 'inherit'
     }));
   };