use Set rather than Map for deshadowing

src/Bundle.js

@@ -212,18 +212,18 @@ export default class Bundle {
 			return name;
 		}
 
-		const toDeshadow = new Map();
+		const toDeshadow = new Set();
 
 		this.externalModules.forEach( module => {
 			const safeName = getSafeName( module.name );
-			toDeshadow.set( safeName, true );
+			toDeshadow.add( safeName );
 			module.name = safeName;
 
 			// ensure we don't shadow named external imports, if
 			// we're creating an ES6 bundle
 			forOwn( module.declarations, ( declaration, name ) => {
 				const safeName = getSafeName( name );
-				toDeshadow.set( safeName, true );
+				toDeshadow.add( safeName );
 				declaration.setSafeName( safeName );
 			});
 		});

src/ast/scopes/ModuleScope.js

@@ -14,13 +14,13 @@ export default class ModuleScope extends Scope {
 	}
 
 	deshadow ( names ) {
-		names = new Map( names );
+		names = new Set( names );
 
 		forOwn( this.module.imports, specifier => {
 			if ( specifier.module.isExternal ) return;
 
 			specifier.module.getExports().forEach( name => {
-				names.set(name);
+				names.add( name );
 			});
 
 			if ( specifier.name !== '*' ) {
@@ -29,9 +29,10 @@ export default class ModuleScope extends Scope {
 					this.module.bundle.onwarn( `Non-existent export '${specifier.name}' is imported from ${specifier.module.id} by ${this.module.id}` );
 					return;
 				}
+
 				const name = declaration.getName( true );
 				if ( name !== specifier.name ) {
-					names.set( declaration.getName( true ) );
+					names.add( declaration.getName( true ) );
 				}
 			}
 		});