Fix ember-cli-babel optimization

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

@@ -27,6 +27,13 @@ function buildCompatAddon(originalPackage: Package, v1Cache: V1InstanceCache): N
 
   let oldPackages = v1Cache.getAddons(originalPackage.root);
 
+  if (oldPackages.length > 1) {
+    // extensibility hook that allows a compat adapter to optimize its own
+    // smooshing. We do it early so that if it reduces all the way to zero, the
+    // next check will handle that.
+    oldPackages = oldPackages[0].reduceInstances(oldPackages);
+  }
+
   if (oldPackages.length === 0) {
     // this happens when the v1 addon wasn't actually getting instantiated at
     // all, which can happen if the app uses `addons.blacklist` or another addon
@@ -39,7 +46,7 @@ function buildCompatAddon(originalPackage: Package, v1Cache: V1InstanceCache): N
     return new EmptyPackageTree(originalPackage.name);
   }
 
-  let needsSmooshing = oldPackages[0].hasAnyTrees();
+  let needsSmooshing = oldPackages.length > 1 && oldPackages[0].hasAnyTrees();
   if (needsSmooshing) {
     let trees = oldPackages.map(pkg => pkg.v2Tree).reverse();
     let smoosher = new SmooshPackageJSON(trees, { annotation: originalPackage.name });

packages/compat/src/compat-adapters/ember-cli-babel.ts

@@ -1,22 +1,18 @@
-import writeFile from 'broccoli-file-creator';
 import V1Addon from '../v1-addon';
 
-// Because almost every addon depends on ember-cli-babel, and because ember-cli
-// instantiates a separate instance of Addon per consumer, approximately *half*
-// of all Addon instances in a typical app will be copies of ember-cli-babel.
-//
-// Under embroider, *all* of them should be contributing no files to the build.
 export default class EmberCliBabel extends V1Addon {
-  // this ensures we don't bother smooshing together a large number of useless
-  // copies of the addon.
-  hasAnyTrees() {
-    return false;
-  }
-
-  // and the one copy that we do emit should just be an empty valid package. We
-  // don't want the babel helpers it emits, they're not even used under
-  // Embroider anyway.
-  get v2Tree() {
-    return writeFile('package.json', JSON.stringify(this.newPackageJSON, null, 2));
+  // the only copy of ember-cli-babel that might need to do something is the
+  // first one that wants to emit babel polyfills. No other copy is allowed to
+  // emit anything into the build.
+  reduceInstances(copies: EmberCliBabel[]): EmberCliBabel[] {
+    let polyfillCopy = copies.find(c => {
+      let instance = c.addonInstance as any;
+      return typeof instance._shouldIncludePolyfill === 'function' && instance._shouldIncludePolyfill();
+    });
+    if (polyfillCopy) {
+      return [polyfillCopy];
+    } else {
+      return [];
+    }
   }
 }

packages/compat/src/v1-addon.ts

@@ -134,6 +134,26 @@ export default class V1Addon {
     }
   }
 
+  // Optional extensible hook for pruning down the list of redundant addon
+  // instances produces by the classic ember-cli architecture. ember-cli
+  // instantiates each addon *per consumer*, not per package. So a given package
+  // will have many addon instances, and Embroider dutifully produces a V1Addon
+  // instance for each one, and then needs to mimic the classic smooshing
+  // behavior between them.
+  //
+  // But some packages (and ember-cli-babel is the motivating example) produce a
+  // huge number of instances that do nothing useful and incur significant cost.
+  // This hook allows their compat adapter to prune down the set, using
+  // addon-specific knowledge of which instance(s) are actually important.
+  //
+  // The order of the instances is significant. The first one is the one with
+  // the highest precedence, meaning its files would win under classic
+  // smooshing.
+  reduceInstances(instances: V1Addon[]): V1Addon[] {
+    // the default beahvior is that all copies matter
+    return instances;
+  }
+
   // this is only defined when there are custom AST transforms that need it
   @Memoize()
   private get templateCompiler(): NodeTemplateCompiler | undefined {