chore(build): Add traceur transpiler for broccoli.

This exactly reproduces the output tree from one of the gulp tasks, which is now removed.
Next step is to migrate another sibling task to broccoli.

tools/broccoli/traceur/index.js

@@ -0,0 +1,50 @@
+var __extends = this.__extends || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    __.prototype = b.prototype;
+    d.prototype = new __();
+};
+var fs = require('fs');
+var fse = require('fs-extra');
+var path = require('path');
+var traceur = require('../../transpiler');
+var walkSync = require('walk-sync');
+var Writer = require('broccoli-writer');
+var xtend = require('xtend');
+var TraceurFilter = (function (_super) {
+    __extends(TraceurFilter, _super);
+    function TraceurFilter(inputTree, options) {
+        if (options === void 0) { options = {}; }
+        this.inputTree = inputTree;
+        this.options = options;
+    }
+    TraceurFilter.prototype.write = function (readTree, destDir) {
+        var _this = this;
+        return readTree(this.inputTree).then(function (srcDir) {
+            walkSync(srcDir).filter(function (filepath) {
+                var extension = path.extname(filepath).toLowerCase();
+                return extension === '.js' || extension === '.es6';
+            }).map(function (filepath) {
+                var options = xtend({
+                    filename: filepath
+                }, _this.options);
+                var fsOpts = {
+                    encoding: 'utf-8'
+                };
+                var sourcecode = fs.readFileSync(path.join(srcDir, filepath), fsOpts);
+                var result = traceur.compile(options, filepath, sourcecode);
+                result.js = result.js + '\n//# sourceMappingURL=./' + path.basename(filepath).replace(/\.\w+$/, '.map');
+                var destFilepath = filepath.replace(/\.\w+$/, '.es6');
+                var destFile = path.join(destDir, destFilepath);
+                fse.mkdirsSync(path.dirname(destFile));
+                var destMap = path.join(destDir, filepath + '.map');
+                fs.writeFileSync(destFile, result.js, fsOpts);
+                result.sourceMap.file = destFilepath;
+                fs.writeFileSync(destMap, JSON.stringify(result.sourceMap), fsOpts);
+            });
+        });
+    };
+    return TraceurFilter;
+})(Writer);
+module.exports = TraceurFilter;
+//# sourceMappingURL=index.js.map