Use broccoli memoization by default

This closes #1112 by removing our OneShot optimization hack in favor of turning on broccoli memoization by default.

Broccoli memoization should absolutely be broccoli's default. It's not yet, but we can turn the flag on here unless users have explicitly turned it off.

packages/compat/package.json

@@ -49,7 +49,6 @@
     "debug": "^4.3.2",
     "fs-extra": "^9.1.0",
     "fs-tree-diff": "^2.0.1",
-    "heimdalljs": "^0.2.6",
     "jsdom": "^16.6.0",
     "lodash": "^4.17.21",
     "pkg-up": "^3.1.0",

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

@@ -3,20 +3,11 @@ import { Package } from '@embroider/core';
 import SmooshPackageJSON from './smoosh-package-json';
 import broccoliMergeTrees from 'broccoli-merge-trees';
 import { Node } from 'broccoli-node-api';
-import OneShot from './one-shot';
 import buildFunnel from 'broccoli-funnel';
 import { UnwatchedDir, WatchedDir } from 'broccoli-source';
 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);
-  }
-  return tree;
-}
-
-function buildCompatAddon(originalPackage: Package, v1Cache: V1InstanceCache): Node {
+export default function buildCompatAddon(originalPackage: Package, v1Cache: V1InstanceCache): Node {
   if (originalPackage.isV2Addon()) {
     // this case is needed when a native-v2 addon depends on a
     // non-native-v2 addon. (The non-native one will get rewritten and

packages/compat/src/compat-addons.ts

@@ -11,6 +11,17 @@ import V1App from './v1-app';
 import TreeSync from 'tree-sync';
 import { WatchedDir } from 'broccoli-source';
 
+// This build stage expects to be run with broccoli memoization enabled in order
+// to get good rebuild performance. We turn it on by default here, but you can
+// still explicitly turn it off by setting the env var to "false".
+//
+// As for safetly mutating process.env: broccoli doesn't read this until a Node
+// executes its build hook, so as far as I can tell there's no way we could set
+// this too late.
+if (typeof process.env.BROCCOLI_ENABLED_MEMOIZE === 'undefined') {
+  process.env.BROCCOLI_ENABLED_MEMOIZE = 'true';
+}
+
 export default class CompatAddons implements Stage {
   private didBuild = false;
   private destDir: string;