feat(compiler-ssr): compile portal (#775)

packages/server-renderer/src/helpers/ssrRenderPortal.ts

@@ -0,0 +1,20 @@
+import { ComponentInternalInstance, ssrContextKey } from 'vue'
+import { SSRContext, createBuffer, PushFn } from '../renderToString'
+
+export function ssrRenderPortal(
+  contentRenderFn: (push: PushFn) => void,
+  target: string,
+  parentComponent: ComponentInternalInstance
+) {
+  const { getBuffer, push } = createBuffer()
+
+  contentRenderFn(push)
+
+  const context = parentComponent.appContext.provides[
+    ssrContextKey as any
+  ] as SSRContext
+  const portalBuffers =
+    context.__portalBuffers || (context.__portalBuffers = {})
+
+  portalBuffers[target] = getBuffer()
+}