Add babel plugin test

packages/babel-preset-default/test/fixtures/polyfill.js

@@ -0,0 +1,6 @@
+// Note: this fixture may need to be updated when the browserslist or the
+// core-js dependencies are updated.
+// It should always test a feature that is supported, but requires
+// a polyfill to work across all supported browsers.
+const foo = new URLSearchParams();
+window.fooSize = foo.size;

packages/babel-preset-default/test/index.js

@@ -24,4 +24,22 @@ describe( 'Babel preset default', () => {
 
 		expect( output.code ).toMatchSnapshot();
 	} );
+
+	test( 'transpilation includes magic comment when using the addPolyfillComments option', () => {
+		const filename = path.join( __dirname, '/fixtures/polyfill.js' );
+		const input = readFileSync( filename );
+
+		const output = transform( input, {
+			filename,
+			configFile: false,
+			envName: 'production',
+			presets: [ babelPresetDefault ],
+			caller: {
+				name: 'WP_BUILD_MAIN',
+				addPolyfillComments: true,
+			},
+		} );
+
+		expect( output.code ).toContain( '/* wp:polyfill */' );
+	} );
 } );