Make side-effectual imports work with globals

lib/graph/treeshake.js

@@ -152,7 +152,11 @@ function loadFromGraph(getNode) {
 }
 
 function sideEffectualModule(node) {
-	return node && node.load.metadata.buildType === "css";
+	if(node) {
+		let md = node.load.metadata;
+		return md.buildType === "css" || md.format === "global";
+	}
+	return false;
 }
 
 function transpile(getNode, data) {

test/tree_shaking_test.js

@@ -92,7 +92,9 @@ describe("Tree-shaking", function(){
 			describe("import 'mod'", function(){
 				it("Includes modules imported for side-effects", function(){
 					assert.equal(browser.window.DEP3_SIDE_EFFECT, true,
-						"Includes a module with needed side effects.");
+						"Includes an ES module with needed side effects.");
+					assert.equal(browser.window.DEP3_SIDE_EFFECT2, true,
+						"Includes a global module with needed side effects.");
 				});
 
 				it("Works when the dependency is CSS", function(){

test/treeshake/basics/main.js

@@ -12,6 +12,7 @@ import steal from "@steal";
 
 // Importing a module for its side effects
 import "dep3";
+import "dep3/global";
 
 // CSS
 import "./styles.css";