Avoid confusion variable reassignments in transformer

Summary:
While trying to find a simple fix for the broken source maps when unit testing (see previous diff), I noticed that control flow in the transformer was unnecessarily complicated.

This doesn't fix the sourcemap issue btw

Reviewed By: passy

Differential Revision: D19158367

fbshipit-source-id: 7dfe4b28eabd4534a32dcb655e534d0f418f0db4

static/transforms/index.js

@@ -19,7 +19,7 @@ function transform({filename, options, src}) {
     options.projectRoot && !__dirname.startsWith(options.projectRoot);
   const isTypeScript = filename.endsWith('.tsx');
 
-  let ast = babylon.parse(src, {
+  const ast = babylon.parse(src, {
     filename,
     plugins: isTypeScript
       ? [
@@ -86,7 +86,7 @@ function transform({filename, options, src}) {
   } else {
     plugins.push(require('./import-react.js'));
   }
-  ast = babel.transformFromAst(ast, src, {
+  const transformed = babel.transformFromAst(ast, src, {
     ast: true,
     babelrc: !filename.includes('node_modules'),
     code: false,
@@ -97,10 +97,10 @@ function transform({filename, options, src}) {
     plugins,
     presets,
     sourceMaps: true,
-  }).ast;
+  });
 
   const result = generate(
-    ast,
+    transformed.ast,
     {
       filename,
       sourceFileName: filename,
@@ -109,7 +109,7 @@ function transform({filename, options, src}) {
     src,
   );
   return {
-    ast,
+    ast: transformed.ast,
     code: result.code,
     filename,
     map: result.map,