Clear Babel Cache and avoid unnecessary AST clone

Summary:
Improves the memory pressure and runtime-performance by cleaning the babel-cache and avoiding to clone the AST in the transformer.

Clearing the babel cache is not strictly necessary because babel uses a `WeakMap` internally. However, we can help the GC since we know that the same AST nodes are no longer referenced (it's either a different file or the file gets reparsed, which results in new nodes).

Before:
https://pxl.cl/1s5nB

After:

https://pxl.cl/1s5nx

Results for building the `MarketplaceHome.entrypoint` on my on-demand using a local metronome package with `--reset-cache`

|        | Run 1  | Run 2  | Run 3  |
|--------|--------|--------|--------|
| Before | 1m 17s | 1m 18s | 1m 19s |
| After  | 1m 14s | 1m 14s | 1m 13s |

Reviewed By: cpojer

Differential Revision: D25175301

fbshipit-source-id: 459074916d1c150a1767d245358e155e81a2661c

packages/metro-transform-worker/src/index.js

@@ -17,7 +17,6 @@ const assetTransformer = require('./utils/assetTransformer');
 const babylon = require('@babel/parser');
 const collectDependencies = require('metro/src/ModuleGraph/worker/collectDependencies');
 const generateImportNames = require('metro/src/ModuleGraph/worker/generateImportNames');
-const nullthrows = require('nullthrows');
 const generate = require('@babel/generator').default;
 const getCacheKey = require('metro-cache-key');
 const getMinifier = require('./utils/getMinifier');
@@ -373,19 +372,18 @@ module.exports = {
 
     plugins.push([inlinePlugin, opts]);
 
-    ast = nullthrows(
-      transformFromAstSync(ast, '', {
-        ast: true,
-        babelrc: false,
-        code: false,
-        configFile: false,
-        comments: false,
-        compact: false,
-        filename,
-        plugins,
-        sourceMaps: false,
-      }).ast,
-    );
+    transformFromAstSync(ast, '', {
+      ast: true,
+      babelrc: false,
+      code: false,
+      configFile: false,
+      comments: false,
+      compact: false,
+      filename,
+      plugins,
+      sourceMaps: false,
+      cloneInputAst: false,
+    });
 
     let dependencyMapName = '';
     let dependencies;

packages/metro/src/DeltaBundler/Worker.js

@@ -13,6 +13,7 @@
 const crypto = require('crypto');
 const fs = require('fs');
 const path = require('path');
+const traverse = require('@babel/traverse').default;
 
 import type {TransformResult} from './types.flow';
 import type {LogEntry} from 'metro-core/src/Logger';
@@ -84,6 +85,14 @@ async function transform(
     transformOptions,
   );
 
+  // The babel cache caches scopes and pathes for already traversed AST nodes.
+  // Clearing the cache here since the nodes of the transformed file are no longer referenced.
+  // This isn't stritcly necessary since the cache uses a WeakMap. However, WeakMap only permit
+  // that unreferenced keys are collected but the values still hold references to the Scope and NodePaths.
+  // Manually clearing the cache allows the GC to collect the Scope and NodePaths without checking if there
+  // exist any other references to the keys.
+  traverse.cache.clear();
+
   const transformFileEndLogEntry = getEndLogEntry(
     transformFileStartLogEntry,
     filename,