allow importing external modules for side effects, fixes #55

src/Module.js

@@ -201,7 +201,7 @@ export default class Module {
 		let strongDependencies = blank();
 
 		this.statements.forEach( statement => {
-			if ( statement.isImportDeclaration && !statement.node.specifiers.length ) {
+			if ( statement.isImportDeclaration && !statement.node.specifiers.length && !statement.module.isExternal ) {
 				// include module for its side-effects
 				strongDependencies[ statement.module.id ] = statement.module; // TODO is this right? `statement.module` should be `this`, surely?
 			}
@@ -475,6 +475,9 @@ export default class Module {
 					return this.bundle.fetchModule( statement.node.source.value, this.id )
 						.then( module => {
 							statement.module = module;
+							if ( module.isExternal ) {
+								return;
+							}
 							return module.markAllStatements();
 						});
 				}

test/function/import-empty-from-external/_config.js

@@ -0,0 +1,15 @@
+module.exports = {
+	description: 'imports external module for side effects',
+	context: {
+		// override require here, making "foo" appear as a global module
+		require: function ( name ) {
+			if ( name === 'foo' ) {
+				return require( './foo' );
+			}
+			return require( name );
+		}
+	},
+	options: {
+		external: [ 'foo' ]
+	}
+};

test/function/import-empty-from-external/foo.js

@@ -0,0 +1 @@
+global.answer = 42;

test/function/import-empty-from-external/main.js

@@ -0,0 +1,3 @@
+import 'foo';
+
+assert.equal( global.answer, 42 );