feat(apiApp): add more warnings (#394)

packages/runtime-core/__tests__/apiApp.spec.ts

@@ -81,6 +81,11 @@ describe('api: createApp', () => {
 
     app.component('BarBaz', () => 'barbaz!')
 
+    app.component('BarBaz', () => 'barbaz!')
+    expect(
+      'Component "BarBaz" has already been registered in target app.'
+    ).toHaveBeenWarnedTimes(1)
+
     const Root = {
       // local override
       components: {
@@ -117,6 +122,13 @@ describe('api: createApp', () => {
       mounted: spy2
     })
 
+    app.directive('BarBaz', {
+      mounted: spy2
+    })
+    expect(
+      'Directive "BarBaz" has already been registered in target app.'
+    ).toHaveBeenWarnedTimes(1)
+
     const Root = {
       // local override
       directives: {
@@ -164,6 +176,7 @@ describe('api: createApp', () => {
       }
     }
     const mixinB = {
+      name: 'mixinB',
       data() {
         return {
           b: 2
@@ -203,6 +216,15 @@ describe('api: createApp', () => {
     app.mixin(mixinA)
     app.mixin(mixinB)
 
+    app.mixin(mixinA)
+    app.mixin(mixinB)
+    expect(
+      'Mixin has already been applied to target app'
+    ).toHaveBeenWarnedTimes(2)
+    expect(
+      'Mixin has already been applied to target app: mixinB'
+    ).toHaveBeenWarnedTimes(1)
+
     const root = nodeOps.createElement('div')
     app.mount(Comp, root)