Use .program template for helpers.

packages/babel-helpers/src/helpers.js

@@ -7,7 +7,7 @@ export default helpers;
 
 // Helpers never include placeholders, so we disable placeholder pattern
 // matching to allow us to use pattern-like variable names.
-const defineHelper = template({ placeholderPattern: false });
+const defineHelper = template.program({ placeholderPattern: false });
 
 helpers.typeof = defineHelper(`
   export default function _typeof(obj) {

packages/babel-helpers/src/index.js

@@ -232,8 +232,7 @@ function loadHelper(name) {
     if (!helpers[name]) throw new ReferenceError(`Unknown helper ${name}`);
 
     const fn = () => {
-      const ast = helpers[name]();
-      return t.file(t.program(Array.isArray(ast) ? ast : [ast]));
+      return t.file(helpers[name]());
     };
 
     const metadata = getHelperMetadata(fn());

packages/babel-helpers/test/helpers/define-helper.js

@@ -17,7 +17,7 @@ export default function defineHelper(
     throw new Error(`The ${id} helper is already defined.`);
   }
   Object.defineProperty(helpers, id, {
-    value: template(code),
+    value: template.program(code),
   });
   return id;
 }