Fix unsafe reuse of broccoli trees in OneShot

packages/compat/src/build-compat-addon.ts

@@ -11,7 +11,7 @@ import EmptyPackageTree from './empty-package-tree';
 export default function cachedBuildCompatAddon(originalPackage: Package, v1Cache: V1InstanceCache): Node {
   let tree = buildCompatAddon(originalPackage, v1Cache);
   if (!originalPackage.mayRebuild) {
-    tree = new OneShot(tree, originalPackage.name);
+    tree = OneShot.create(tree, originalPackage.name);
   }
   return tree;
 }

packages/compat/src/one-shot.ts

@@ -17,12 +17,23 @@ class NerfHeimdallBuilder extends Builder {
   buildHeimdallTree() {}
 }
 
+let seen = new WeakMap<Node, Node>();
+
 // Wraps a broccoli tree such that it (and everything it depends on) will only
 // build a single time.
 export default class OneShot extends Plugin {
   private builder: NerfHeimdallBuilder | null;
 
-  constructor(originalTree: Node, private addonName: string) {
+  static create(originalTree: Node, privateAddonName: string) {
+    let output = seen.get(originalTree);
+    if (!output) {
+      output = new this(originalTree, privateAddonName);
+      seen.set(originalTree, output);
+    }
+    return output;
+  }
+
+  private constructor(originalTree: Node, private addonName: string) {
     // from broccoli's perspective, we don't depend on any input trees!
     super([], {
       annotation: `@embroider/compat: ${addonName}`,