Use declarations, regardless of whether they have statements or not

Fixes #204

src/Bundle.js

@@ -58,7 +58,7 @@ export default class Bundle {
 					const declaration = entryModule.traceExport( name );
 					declaration.isExported = true;
 
-					if ( declaration.statement ) declaration.use();
+					declaration.use();
 				});
 
 				let settled = false;

test/function/namespaces-are-frozen/main.js

@@ -1,5 +1,3 @@
-// import * as ns from './mod';
+import * as ns from './mod';
 
-// export { ns };
+export { ns };
-
-export var ns = Object.freeze({ a: 1, b: 2 });