Add parser support for placeholders (#9364)

packages/babel-parser/src/plugin-utils.js

@@ -88,12 +88,17 @@ import estree from "./plugins/estree";
 import flow from "./plugins/flow";
 import jsx from "./plugins/jsx";
 import typescript from "./plugins/typescript";
+import placeholders from "./plugins/placeholders";
 
-// NOTE: estree must load first; flow and typescript must load last.
-export const mixinPluginNames = ["estree", "jsx", "flow", "typescript"];
+// NOTE: order is important. estree must come first; placeholders must come last.
 export const mixinPlugins: { [name: string]: MixinPlugin } = {
   estree,
   jsx,
   flow,
   typescript,
+  placeholders,
 };
+
+export const mixinPluginNames: $ReadOnlyArray<string> = Object.keys(
+  mixinPlugins,
+);