Enable traversing non-ES modules imports

This makes it so that if an ES module is only imported by non-ES
modules, it is still included in the tree-shaking algorithm and can be
tree-shaken.

Fixes #1068

lib/graph/treeshake.js

@@ -112,29 +112,36 @@ function loadFromGraph(getNode) {
 					}
 				}
 
+				let code = '';
+
+				if(node) {
+					// Add deps as regular imports
+					for(let specifier of node.load.metadata.deps) {
+						code += `import "${specifier}";\n`;
+					}
+				}
+
 				// Expose named exports so that dependant modules will tree-shake properly.
 				if(needToExport.size) {
-					let code = '';
 					for(let exp of needToExport) {
 						if(exp === "default") {
 							code += "export default {};\n";
 						} else {
 							code += `export let ${exp} = {};\n`;
 						}
 					}
-
-					return code;
 				} else {
 					// Prevent tree shaking modules that are side-effectual like CSS
 					if(sideEffectualModule(node)) {
-						return `
+						code += `
 							export function one() {window.ONE = {}};
 							one();
 						`.trim();
+					} else {
+						code += "export default {}";
 					}
-
-					return "export default {}";
 				}
+				return code;
 			}
 
 			return source.node(node);

test/tree_shaking_test.js

@@ -127,7 +127,7 @@ describe("Tree-shaking", function(){
 			});
 		});
 
-		describe.only("ES modules imported by CommonJS modules", function() {
+		describe("ES modules imported by CommonJS modules", function() {
 			it("Tree-shakes their ES module dependencies", function() {
 				let mod = app.dep5dep5AnotherESModule;
 				assert.equal(typeof mod.mainThing, "function", "Kept the used export");