fix(custom-element): fix event listeners with capital letter event names on custom elements

close https://github.com/vuejs/docs/issues/1708
close https://github.com/vuejs/docs/pull/1890

packages/vue/__tests__/customElementCasing.spec.ts

@@ -0,0 +1,42 @@
+import { createApp } from '../src'
+
+// https://github.com/vuejs/docs/pull/1890
+// https://github.com/vuejs/core/issues/5401
+// https://github.com/vuejs/docs/issues/1708
+test('custom element event casing', () => {
+  customElements.define(
+    'custom-event-casing',
+    class Foo extends HTMLElement {
+      connectedCallback() {
+        this.dispatchEvent(new Event('camelCase'))
+        this.dispatchEvent(new Event('CAPScase'))
+        this.dispatchEvent(new Event('PascalCase'))
+      }
+    }
+  )
+
+  const container = document.createElement('div')
+  document.body.appendChild(container)
+
+  const handler = jest.fn()
+  const handler2 = jest.fn()
+  createApp({
+    template: `
+    <custom-event-casing
+      @camelCase="handler"
+      @CAPScase="handler"
+      @PascalCase="handler"
+      v-on="{
+        camelCase: handler2,
+        CAPScase: handler2,
+        PascalCase: handler2
+      }" />`,
+    methods: {
+      handler,
+      handler2
+    }
+  }).mount(container)
+
+  expect(handler).toHaveBeenCalledTimes(3)
+  expect(handler2).toHaveBeenCalledTimes(3)
+})