Add option to keep ES2015 code in built code

Instead of always forcing code to be transpiled to ES5

lib/graph/transpile.js

@@ -47,7 +47,9 @@ var transpileNode = function(node, outputFormat, options, graph, loader){
 		}
 
 		transformActiveSource(node, key(options, outputFormat), function(node, source){
-			var opts = Object.assign({}, options);
+			var opts = Object.assign({}, options, {
+				forceES5: loader.forceES5 === true
+			});
 			var depMap = nodeDependencyMap(node);
 
 			// this options is used by steal-tools transform

package.json

@@ -31,14 +31,14 @@
     "prettier": "1.12.0",
     "pump": "^3.0.0",
     "rollup-plugin-commonjs": "^9.1.3",
-    "steal": "^2.1.10",
+    "steal": "stealjs/steal#keep-es2015",
     "steal-bundler": "^0.3.6",
     "steal-parse-amd": "^1.0.0",
     "steal-rollup": "^0.58.4",
     "through2": "^3.0.0",
     "tmp": "0.0.33",
     "traceur": "0.0.111",
-    "transpile": "^2.6.1",
+    "transpile": "stealjs/transpile#force-es5",
     "uglify-es": "3.1.3",
     "urix": "^0.1.0",
     "winston": "^2.2.0",

test/keep_es2015/config.js

@@ -0,0 +1,4 @@
+steal.config({
+	forceES5: false,
+	treeShaking: false
+});

test/keep_es2015/main.js

@@ -0,0 +1,3 @@
+async function callMe() {}
+
+export default callMe;

test/multibuild_test.js

@@ -2304,4 +2304,30 @@ describe("multi build", function(){
 			done();
 		}, done).catch(done);
 	});
+
+	it("skips ES2015 transpilation if data.loader.forceES5 is false", function() {
+		var config = {
+			config: path.join(__dirname, "keep_es2015", "config.js"),
+			main: "main"
+		};
+
+		var options = {
+			minify: false,
+			quiet: true
+		};
+
+		return asap(rmdir)(path.join(__dirname, "keep_es2015", "dist"))
+			.then(function() {
+				return multiBuild(config, options);
+			})
+			.then(function() {
+				var main = path.join(__dirname, "keep_es2015", "dist", "bundles", "main.js");
+				var actualJS = fs.readFileSync(main, "utf8");
+
+				assert(
+					actualJS.includes("async function callMe()"),
+					"source should include async function untranspiled"
+				);
+			});
+	});
 });