assume functions can mutate their arguments

src/ast/analyse.js

@@ -145,19 +145,27 @@ export default function analyse ( ast, magicString, module ) {
 		}
 
 		function checkForWrites ( node ) {
-			if ( node.type === 'AssignmentExpression' ) {
-				let assignee = node.left;
-
-				while ( assignee.type === 'MemberExpression' ) {
-					assignee = assignee.object;
+			function addNode ( node ) {
+				while ( node.type === 'MemberExpression' ) {
+					node = node.object;
 				}
 
-				if ( assignee.type !== 'Identifier' ) { // could be a ThisExpression
+				if ( node.type !== 'Identifier' ) {
 					return;
 				}
 
-				statement._modifies[ assignee.name ] = true;
+				statement._modifies[ node.name ] = true;
+			}
+
+			if ( node.type === 'AssignmentExpression' ) {
+				addNode( node.left );
+			}
+
+			else if ( node.type === 'CallExpression' ) {
+				node.arguments.forEach( addNode );
 			}
+
+			// TODO UpdateExpression
 		}
 
 		walk( statement, {

test/samples/function-side-effects/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'assumes functions may mutate objects passed in'
+};

test/samples/function-side-effects/main.js

@@ -0,0 +1,3 @@
+import object from './object';
+assert.ok( object.mutated );
+

test/samples/function-side-effects/mutate.js

@@ -0,0 +1,3 @@
+export default function mutate ( object ) {
+	object.mutated = true;
+}

test/samples/function-side-effects/object.js

@@ -0,0 +1,6 @@
+import mutate from './mutate';
+
+var object = {};
+mutate( object );
+
+export default object;