Make ember-lts-4.12 compat scenario runnable

Two stacked runtime issues were blocking ember-lts-4.12 once the
`require` import issue from #321 cleared. Both only affect the compat
path (ENABLE_COMPAT_BUILD=true, ember4 deps overrides) and are hidden
for ember-source >= 6 because the upstream code was rewritten.

1. "Cannot read properties of undefined (reading 'has')" at
   `makeComputedDecorator` → `COMPUTED_GETTERS.has(...)`.

   ember-source <= 5.x's `@ember/-internals/metal` has
   `let COMPUTED_GETTERS; if (DEBUG) { COMPUTED_GETTERS = new WeakSet(); }`
   paired with `assert(..., !COMPUTED_GETTERS.has(...))`. `DEBUG` comes
   from `@glimmer/env`, whose published default is `DEBUG = false`.
   Without something resolving `DEBUG` at compile time, rollup tree-shakes
   the init but the `assert` predicate still runs eagerly, so
   `COMPUTED_GETTERS` is undefined at runtime.

   Conditionally add `babel-plugin-debug-macros` (only when
   ENABLE_COMPAT_BUILD is set) with `flags: [{ source: '@glimmer/env',
   flags: { DEBUG: true } }]` so the plugin inlines `DEBUG = true` in
   ember-source. The init and the use stay consistent. Non-compat
   scenarios don't need the plugin — modern ember-source uses
   `isDevelopingApp()` from `@embroider/macros`, which `buildMacros()`
   already handles.

2. ember-qunit@8 + @ember/test-helpers@4 rely on classic-Ember globals
   that don't exist in a vite build: `start()` auto-calls `loadTests()`
   which iterates `requirejs.entries`, and `visit` reads
   `global.EmberENV._APPLICATION_TEMPLATE_WRAPPER`. Both were removed in
   ember-qunit@9 and @ember/test-helpers@5 respectively. Bump those two
   pins in the ember4 deps stanza so the scenario exercises the same
   surface modern scenarios do:

       ember-qunit:          ^8.0.0  ->  ^9.0.0
       @ember/test-helpers:  ^4.0.0  ->  ^5.0.0

   Both peer `ember-source >= 4.0.0`, so Ember 4.12 is still within the
   supported range. ember-qunit@9 peers `@ember/test-helpers >=3.0.3`,
   so the test-helpers bump stays within bounds.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: NullVoxPopuli

pnpm-lock.yaml

@@ -17,10 +17,10 @@ module.exports = async function (defaults) {
 };
 
   const ember4 = {
-    '@ember/test-helpers': '^4.0.0',
+    '@ember/test-helpers': '^5.0.0',
     '@ember/test-waiters': '^3.1.0',
     '@embroider/compat': '^4.0.3',
-    'ember-qunit': '^8.0.0',
+    'ember-qunit': '^9.0.0',
     'ember-cli': '~4.12.0',
   };
 

test-app/.try.mjs

@@ -4,6 +4,8 @@ import { buildMacros } from '@embroider/macros/babel';
 
 const macros = buildMacros();
 
+const isCompatBuild = !!process.env.ENABLE_COMPAT_BUILD;
+
 export default {
   plugins: [
     [
@@ -44,6 +46,16 @@ export default {
       },
     ],
     ...macros.babelMacros,
+    ...(isCompatBuild
+      ? [
+          [
+            'babel-plugin-debug-macros',
+            {
+              flags: [{ source: '@glimmer/env', flags: { DEBUG: true } }],
+            },
+          ],
+        ]
+      : []),
   ],
 
   generatorOpts: {

test-app/babel.config.mjs

@@ -35,6 +35,7 @@
     "@babel/eslint-parser": "^7.23.3",
     "@babel/plugin-transform-runtime": "^7.29.0",
     "@babel/plugin-transform-typescript": "^7.28.6",
+    "babel-plugin-debug-macros": "^0.3.4",
     "@ember/string": "^4.0.1",
     "@ember/test-helpers": "^5.4.1",
     "@embroider/core": "^4.4.7",