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

tools/build/util.js

@@ -0,0 +1,45 @@
+var Q = require('q');
+var minimatch = require('minimatch');
+
+module.exports = {
+  processToPromise: processToPromise,
+  streamToPromise: streamToPromise,
+  renameSrcFolder: renameSrcFolder,
+  filterByFile: filterByFile
+};
+
+function filterByFile(valuesWithPatterns, fileName) {
+  var match = null;
+  for (var pattern in valuesWithPatterns) {
+    if (pattern !== 'default' && minimatch(fileName, pattern)) {
+      match = valuesWithPatterns[pattern];
+    }
+  }
+  return match || valuesWithPatterns['default'];
+}
+
+function processToPromise(process) {
+  var defer = Q.defer();
+  process.on('close', function(code) {
+    if (code) {
+      defer.reject(code);
+    } else {
+      defer.resolve();
+    }
+  });
+  return defer.promise;
+}
+
+function streamToPromise(stream) {
+  var defer = Q.defer();
+  stream.on('end', defer.resolve);
+  stream.on('error', defer.reject);
+  return defer.promise;
+}
+
+function renameSrcFolder(plugins, srcFolderMapping) {
+  return plugins.rename(function(file) {
+    var srcOutputFolder = filterByFile(srcFolderMapping, file.dirname);
+    file.dirname = file.dirname.replace(/\bsrc\b/, srcOutputFolder);
+  });
+}