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/dartdetect.js

@@ -0,0 +1,33 @@
+var which = require('which');
+
+module.exports = function(gulp) {
+  var DART_SDK = false;
+  try {
+    which.sync('dart');
+    console.log('Dart SDK detected');
+    if (process.platform === 'win32') {
+      DART_SDK = {
+        PUB: 'pub.bat',
+        ANALYZER: 'dartanalyzer.bat'
+      };
+    } else {
+      DART_SDK = {
+        PUB: 'pub',
+        ANALYZER: 'dartanalyzer'
+      };
+    }
+  } catch (e) {
+    console.log('Dart SDK is not available, Dart tasks will be skipped.');
+    var gulpTaskFn = gulp.task.bind(gulp);
+    gulp.task = function (name, deps, fn) {
+      if (name.indexOf('.dart') === -1) {
+        return gulpTaskFn(name, deps, fn);
+      } else {
+        return gulpTaskFn(name, function() {
+          console.log('Dart SDK is not available. Skipping task: ' + name);
+        });
+      }
+    };
+  }
+  return DART_SDK;
+}