Add failing test

test/function/handles-stringified-sourcemaps/_config.js

@@ -0,0 +1,18 @@
+module.exports = {
+	options: {
+		plugins: [
+			{
+				transform: function ( code ) {
+					return {
+						code: code,
+						map: JSON.stringify({ mappings: '' })
+					};
+				}
+			}
+		]
+	},
+
+	bundleOptions: {
+		sourceMap: true
+	}
+};

test/function/handles-stringified-sourcemaps/main.js

@@ -0,0 +1 @@
+export default 42;