test(compiler-cli): load test files into memory only once (#38909)

Prior to this change, each invocation of `loadStandardTestFiles` would
load the necessary files from disk. This function is typically called
at the top-level of a test module in order to share the result across
tests. The `//packages/compiler-cli/test/ngtsc` target has 8 modules
where this call occurs, each loading their own copy of
`node_modules/typescript` which is ~60MB in size, so the memory overhead
used to be significant. This commit loads the individual packages into
a standalone `Folder` and mounts this folder into the filesystem of
standard test files, such that all file contents are no longer
duplicated in memory.

PR Close #38909

packages/compiler-cli/test/helpers/src/mock_file_loading.ts

@@ -22,28 +22,43 @@ export function loadTestFiles(files: TestFile[]) {
   });
 }
 
+/**
+ * A folder that is lazily loaded upon first access and then cached.
+ */
+class CachedFolder {
+  private folder: Folder|null = null;
+
+  constructor(private loader: () => Folder) {}
+
+  get(): Folder {
+    if (this.folder === null) {
+      this.folder = this.loader();
+    }
+    return this.folder;
+  }
+}
+
+const typescriptFolder = new CachedFolder(() => loadFolder(resolveNpmTreeArtifact('typescript')));
+const angularFolder = new CachedFolder(loadAngularFolder);
+const rxjsFolder = new CachedFolder(() => loadFolder(resolveNpmTreeArtifact('rxjs')));
+
 export function loadStandardTestFiles(
     {fakeCore = true, rxjs = false}: {fakeCore?: boolean, rxjs?: boolean} = {}): Folder {
   const tmpFs = new MockFileSystemPosix(true);
   const basePath = '/' as AbsoluteFsPath;
 
-  loadTestDirectory(
-      tmpFs, resolveNpmTreeArtifact('typescript'),
-      tmpFs.resolve(basePath, 'node_modules/typescript'));
+  tmpFs.mount(tmpFs.resolve('/node_modules/typescript'), typescriptFolder.get());
 
   loadTsLib(tmpFs, basePath);
 
   if (fakeCore) {
     loadFakeCore(tmpFs, basePath);
   } else {
-    getAngularPackagesFromRunfiles().forEach(({name, pkgPath}) => {
-      loadTestDirectory(tmpFs, pkgPath, tmpFs.resolve(basePath, 'node_modules/@angular', name));
-    });
+    tmpFs.mount(tmpFs.resolve('/node_modules/@angular'), angularFolder.get());
   }
 
   if (rxjs) {
-    loadTestDirectory(
-        tmpFs, resolveNpmTreeArtifact('rxjs'), tmpFs.resolve(basePath, 'node_modules/rxjs'));
+    tmpFs.mount(tmpFs.resolve('/node_modules/rxjs'), rxjsFolder.get());
   }
 
   return tmpFs.dump();
@@ -60,6 +75,20 @@ export function loadFakeCore(fs: FileSystem, basePath: string = '/') {
       fs.resolve(basePath, 'node_modules/@angular/core'));
 }
 
+function loadFolder(path: string): Folder {
+  const tmpFs = new MockFileSystemPosix(true);
+  loadTestDirectory(tmpFs, tmpFs.resolve(path), tmpFs.resolve('/'));
+  return tmpFs.dump();
+}
+
+function loadAngularFolder(): Folder {
+  const tmpFs = new MockFileSystemPosix(true);
+  getAngularPackagesFromRunfiles().forEach(({name, pkgPath}) => {
+    loadTestDirectory(tmpFs, pkgPath, tmpFs.resolve(name));
+  });
+  return tmpFs.dump();
+}
+
 /**
  * Load real files from the real file-system into a mock file-system.
  * @param fs the file-system where the directory is to be loaded.