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

packages/runtime-core/src/apiApp.ts

@@ -110,7 +110,19 @@ export function createAppAPI<HostNode, HostElement>(
       },
 
       mixin(mixin: ComponentOptions) {
-        context.mixins.push(mixin)
+        if (__DEV__ && !__FEATURE_OPTIONS__) {
+          warn('Mixins are only available in builds supporting Options API')
+        }
+
+        if (!context.mixins.includes(mixin)) {
+          context.mixins.push(mixin)
+        } else if (__DEV__) {
+          warn(
+            'Mixin has already been applied to target app' +
+              (mixin.name ? `: ${mixin.name}` : '')
+          )
+        }
+
         return app
       },
 
@@ -121,6 +133,11 @@ export function createAppAPI<HostNode, HostElement>(
         if (!component) {
           return context.components[name]
         } else {
+          if (__DEV__ && context.components[name]) {
+            warn(
+              `Component "${name}" has already been registered in target app.`
+            )
+          }
           context.components[name] = component
           return app
         }
@@ -134,6 +151,11 @@ export function createAppAPI<HostNode, HostElement>(
         if (!directive) {
           return context.directives[name] as any
         } else {
+          if (__DEV__ && context.directives[name]) {
+            warn(
+              `Directive "${name}" has already been registered in target app.`
+            )
+          }
           context.directives[name] = directive
           return app
         }