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/dartanalyzer.js

@@ -0,0 +1,70 @@
+var Q = require('q');
+var readline = require('readline');
+var spawn = require('child_process').spawn;
+var path = require('path');
+var glob = require('glob');
+var fs = require('fs');
+var util = require('./util');
+
+module.exports = function(gulp, plugins, config) {
+  return function() {
+    var dartModuleFolders = [].slice.call(glob.sync(config.dest + '/*'));
+    var tempFile = '_analyzer.dart';
+    // analyze in parallel!
+    return Q.all(dartModuleFolders.map(function(dir) {
+      var srcFiles = [].slice.call(glob.sync(util.filterByFile(config.srcFolderMapping, dir) + '/**/*.dart', {
+        cwd: dir
+      }));
+      var testFiles = [].slice.call(glob.sync('test/**/*_spec.dart', {
+        cwd: dir
+      }));
+      var analyzeFile = ['library _analyzer;'];
+      srcFiles.concat(testFiles).forEach(function(fileName, index) {
+        if (fileName !== tempFile) {
+          analyzeFile.push('import "./'+fileName+'" as mod'+index+';');
+        }
+      });
+      fs.writeFileSync(path.join(dir, tempFile), analyzeFile.join('\n'));
+      var defer = Q.defer();
+      analyze(dir, defer.makeNodeResolver());
+      return defer.promise;
+    }));
+
+    function analyze(dirName, done) {
+      var stream = spawn(config.command, ['--fatal-warnings', tempFile], {
+        // inherit stdin and stderr, but filter stdout
+        stdio: [process.stdin, 'pipe', process.stderr],
+        cwd: dirName
+      });
+      // Filter out unused imports from our generated file.
+      // We don't reexports from the generated file
+      // as this could lead to name clashes when two files
+      // export the same thing.
+      var rl = readline.createInterface({
+        input: stream.stdout,
+        output: process.stdout,
+        terminal: false
+      });
+      var hintCount = 0;
+      rl.on('line', function(line) {
+        if (line.match(/Unused import/)) {
+          return;
+        }
+        if (line.match(/\[hint\]/)) {
+          hintCount++;
+        }
+        console.log(dirName + ':' + line);
+      });
+      stream.on('close', function(code) {
+        var error;
+        if (code !== 0) {
+          error = new Error('Dartanalyzer failed with exit code ' + code);
+        }
+        if (hintCount > 0) {
+          error = new Error('Dartanalyzer showed hints');
+        }
+        done(error);
+      });
+    }
+  };
+};