fix(migrations): preserve tsconfig in standalone migration (#48987)

For the standalone migration we need to pass a couple of compiler flags which accidentally also overwrote the project's compiler options. These changes extend the options instead.

PR Close #48987

Author: crisbeto

packages/core/schematics/ng-generate/standalone-migration/index.ts

@@ -61,18 +61,19 @@ export default function(options: Options): Rule {
 
 function standaloneMigration(
     tree: Tree, tsconfigPath: string, basePath: string, pathToMigrate: string,
-    options: Options): number {
-  if (options.path.startsWith('..')) {
+    schematicOptions: Options): number {
+  if (schematicOptions.path.startsWith('..')) {
     throw new SchematicsException(
         'Cannot run standalone migration outside of the current project.');
   }
 
-  const {host, rootNames} = createProgramOptions(tree, tsconfigPath, basePath);
-  const program = createProgram({
-                    rootNames,
-                    host,
-                    options: {_enableTemplateTypeChecker: true, compileNonExportedClasses: true}
-                  }) as NgtscProgram;
+  const {host, options, rootNames} = createProgramOptions(
+      tree, tsconfigPath, basePath, undefined, undefined,
+      {
+        _enableTemplateTypeChecker: true,  // Required for the template type checker to work.
+        compileNonExportedClasses: true,   // We want to migrate non-exported classes too.
+      });
+  const program = createProgram({rootNames, host, options}) as NgtscProgram;
   const printer = ts.createPrinter();
 
   if (existsSync(pathToMigrate) && !statSync(pathToMigrate).isDirectory()) {
@@ -92,11 +93,11 @@ function standaloneMigration(
   let pendingChanges: ChangesByFile;
   let filesToRemove: Set<ts.SourceFile>|null = null;
 
-  if (options.mode === MigrationMode.pruneModules) {
+  if (schematicOptions.mode === MigrationMode.pruneModules) {
     const result = pruneNgModules(program, host, basePath, rootNames, sourceFiles, printer);
     pendingChanges = result.pendingChanges;
     filesToRemove = result.filesToRemove;
-  } else if (options.mode === MigrationMode.standaloneBootstrap) {
+  } else if (schematicOptions.mode === MigrationMode.standaloneBootstrap) {
     pendingChanges =
         toStandaloneBootstrap(program, host, basePath, rootNames, sourceFiles, printer);
   } else {

packages/core/schematics/utils/typescript/compiler_host.ts

@@ -39,17 +39,19 @@ export function createMigrationProgram(
  * @param fakeFileRead Optional file reader function. Can be used to overwrite files in
  *   the TypeScript program, or to add in-memory files (e.g. to add global types).
  * @param additionalFiles Additional file paths that should be added to the program.
+ * @param optionOverrides Overrides of the parsed compiler options.
  */
 export function createProgramOptions(
     tree: Tree, tsconfigPath: string, basePath: string, fakeFileRead?: FakeReadFileFn,
-    additionalFiles?: string[]) {
+    additionalFiles?: string[], optionOverrides?: ts.CompilerOptions) {
   // Resolve the tsconfig path to an absolute path. This is needed as TypeScript otherwise
   // is not able to resolve root directories in the given tsconfig. More details can be found
   // in the following issue: https://github.com/microsoft/TypeScript/issues/37731.
   tsconfigPath = resolve(basePath, tsconfigPath);
   const parsed = parseTsconfigFile(tsconfigPath, dirname(tsconfigPath));
-  const host = createMigrationCompilerHost(tree, parsed.options, basePath, fakeFileRead);
-  return {rootNames: parsed.fileNames.concat(additionalFiles || []), options: parsed.options, host};
+  const options = optionOverrides ? {...parsed.options, ...optionOverrides} : parsed.options;
+  const host = createMigrationCompilerHost(tree, options, basePath, fakeFileRead);
+  return {rootNames: parsed.fileNames.concat(additionalFiles || []), options, host};
 }
 
 function createMigrationCompilerHost(