Ensure `delete` operations at the top level are preserved.

Fixes #352.

src/Statement.js

@@ -1,7 +1,7 @@
 import { walk } from 'estree-walker';
 import Scope from './ast/Scope.js';
 import attachScopes from './ast/attachScopes.js';
-import modifierNodes from './ast/modifierNodes.js';
+import modifierNodes, { isModifierNode } from './ast/modifierNodes.js';
 import isFunctionDeclaration from './ast/isFunctionDeclaration.js';
 import isReference from './ast/isReference.js';
 import getLocation from './utils/getLocation.js';
@@ -95,7 +95,7 @@ export default class Statement {
 
 				let isReassignment;
 
-				if ( parent && parent.type in modifierNodes ) {
+				if ( parent && isModifierNode( parent ) ) {
 					let subject = parent[ modifierNodes[ parent.type ] ];
 					let depth = 0;
 

src/ast/modifierNodes.js

@@ -1,4 +1,19 @@
-export default {
+const modifierNodes = {
 	AssignmentExpression: 'left',
-	UpdateExpression: 'argument'
+	UpdateExpression: 'argument',
+	UnaryExpression: 'argument'
 };
+
+export default modifierNodes;
+
+export function isModifierNode ( node ) {
+	if ( !( node.type in modifierNodes ) ) {
+		return false;
+	}
+
+	if ( node.type === 'UnaryExpression' ) {
+		return node.operator === 'delete';
+	}
+
+	return true;
+}

src/utils/run.js

@@ -1,5 +1,5 @@
 import { walk } from 'estree-walker';
-import modifierNodes from '../ast/modifierNodes.js';
+import modifierNodes, { isModifierNode } from '../ast/modifierNodes.js';
 import isReference from '../ast/isReference.js';
 import flatten from '../ast/flatten';
 
@@ -115,7 +115,7 @@ export default function run ( node, scope, statement, strongDependencies, force
 				}
 			}
 
-			else if ( node.type in modifierNodes ) {
+			else if ( isModifierNode( node ) ) {
 				let subject = node[ modifierNodes[ node.type ] ];
 				while ( subject.type === 'MemberExpression' ) subject = subject.object;
 

test/function/delete/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+  description: '`delete` operator at the top level is preserved'
+};

test/function/delete/main.js

@@ -0,0 +1,4 @@
+var a = { b: 1 };
+assert.strictEqual(a.b, 1);
+delete a.b;
+assert.strictEqual(a.b, undefined);