fix(v-model): allow v-model usage on declared custom elements

fix #1699

packages/compiler-dom/__tests__/transforms/__snapshots__/vModel.spec.ts.snap

@@ -1,5 +1,21 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`compiler: transform v-model errors should allow custom element 1`] = `
+"const _Vue = Vue
+
+return function render(_ctx, _cache) {
+  with (_ctx) {
+    const { vModelText: _vModelText, createVNode: _createVNode, withDirectives: _withDirectives, openBlock: _openBlock, createBlock: _createBlock } = _Vue
+
+    return _withDirectives((_openBlock(), _createBlock(\\"my-input\\", {
+      \\"onUpdate:modelValue\\": $event => (model = $event)
+    }, null, 8 /* PROPS */, [\\"onUpdate:modelValue\\"])), [
+      [_vModelText, model]
+    ])
+  }
+}"
+`;
+
 exports[`compiler: transform v-model input w/ dynamic v-bind 1`] = `
 "const _Vue = Vue
 

packages/compiler-dom/__tests__/transforms/vModel.spec.ts

@@ -115,6 +115,17 @@ describe('compiler: transform v-model', () => {
       )
     })
 
+    test('should allow usage on custom element', () => {
+      const onError = jest.fn()
+      const root = transformWithModel('<my-input v-model="model" />', {
+        onError,
+        isCustomElement: tag => tag.startsWith('my-')
+      })
+      expect(root.helpers).toContain(V_MODEL_TEXT)
+      expect(onError).not.toHaveBeenCalled()
+      expect(generate(root).code).toMatchSnapshot()
+    })
+
     test('should raise error if used file input element', () => {
       const onError = jest.fn()
       transformWithModel(`<input type="file" v-model="test"/>`, {