test: add test case for #1081 (custom block default export)

Author: yyx990803

test/fixtures/custom-blocks.vue

@@ -7,6 +7,12 @@ ja:
 
 <blog>## foo</blog>
 
+<esm>
+export default function (Component) {
+  Component.options.foo = 1
+}
+</esm>
+
 <template>
   <div>
     <h1>{{ msg }}</h1>

test/mock-loaders/identity.js

@@ -0,0 +1,3 @@
+module.exports = function (source, map) {
+  this.callback(null, source, map)
+}

test/test.js

@@ -782,6 +782,21 @@ describe('vue-loader', () => {
     })
   })
 
+  it('custom blocks with ES module default export', done => {
+    test({
+      entry: './test/fixtures/custom-blocks.vue',
+      vue: {
+        loaders: {
+          esm: require.resolve('./mock-loaders/identity')
+        }
+      }
+    }, (window, module) => {
+      // option added by custom block code
+      expect(module.foo).to.equal(1)
+      done()
+    })
+  })
+
   it('passes attributes as options to the loader', done => {
     bundle({
       entry: './test/fixtures/custom-options.vue',