refactor(@angular/build): use standalone Instrumenter API for code coverage

This refactors the code coverage instrumentation in the esbuild pipeline to bypass Babel and use the standalone `Instrumenter` API from `istanbul-lib-instrument` directly.
The instrumenter is only used by the Karma/Jasmine testing implementation.

Since first-party code coverage instrumentation and third-party Angular linking are almost always mutually exclusive for any given file, we do not benefit from composing them into a single Babel pass. Using the high-level standalone API simplifies the transformer worker and allows us to completely remove the custom `add-code-coverage.ts` Babel plugin.

Author: clydin

packages/angular/build/src/tools/babel/plugins/add-code-coverage.ts

@@ -7,14 +7,14 @@
  */
 
 declare module 'istanbul-lib-instrument' {
-  export interface Visitor {
-    enter(path: import('@babel/core').NodePath<types.Program>): void;
-    exit(path: import('@babel/core').NodePath<types.Program>): void;
+  export interface Instrumenter {
+    instrumentSync(code: string, filename: string, inputSourceMap?: object): string;
+    lastSourceMap(): object | undefined;
   }
 
-  export function programVisitor(
-    types: typeof import('@babel/core').types,
-    filePath?: string,
-    options?: { inputSourceMap?: object | null },
-  ): Visitor;
+  export function createInstrumenter(options?: {
+    produceSourceMap?: boolean;
+    esModules?: boolean;
+    coverageVariable?: string;
+  }): Instrumenter;
 }

packages/angular/build/src/tools/babel/plugins/types.d.ts

@@ -36,6 +36,46 @@ const textEncoder = new TextEncoder();
  */
 const LINKER_DECLARATION_PREFIX = 'ɵɵngDeclare';
 
+async function instrumentCoverage(
+  filename: string,
+  data: string,
+  useInputSourcemap: boolean,
+): Promise<string> {
+  try {
+    let resolvedPath = 'istanbul-lib-instrument';
+    try {
+      const requireFn = createRequire(filename);
+      resolvedPath = requireFn.resolve('istanbul-lib-instrument');
+    } catch {
+      // Fallback to pool worker import traversal
+    }
+
+    const { createInstrumenter } = (await import(
+      resolvedPath
+    )) as typeof import('istanbul-lib-instrument');
+    const instrumenter = createInstrumenter({
+      produceSourceMap: useInputSourcemap,
+      esModules: true,
+    });
+
+    const instrumentedCode = instrumenter.instrumentSync(data, filename);
+    const lastMap = instrumenter.lastSourceMap();
+
+    if (useInputSourcemap && lastMap) {
+      const inlineMap = Buffer.from(JSON.stringify(lastMap)).toString('base64');
+
+      return instrumentedCode + `\n//# sourceMappingURL=data:application/json;base64,${inlineMap}`;
+    }
+
+    return removeSourceMappingURL(instrumentedCode);
+  } catch (error) {
+    throw new Error(
+      `The 'istanbul-lib-instrument' package is required for code coverage but was not found. Please install the package.`,
+      { cause: error },
+    );
+  }
+}
+
 export default async function transformJavaScript(
   request: JavaScriptTransformRequest,
 ): Promise<unknown> {
@@ -59,41 +99,18 @@ async function transformWithBabel(
   data: string,
   options: Omit<JavaScriptTransformRequest, 'filename' | 'data'>,
 ): Promise<string> {
-  const shouldLink = !options.skipLinker && (await requiresLinking(filename, data));
   const useInputSourcemap =
     options.sourcemap &&
     (!!options.thirdPartySourcemaps || !/[\\/]node_modules[\\/]/.test(filename));
 
-  const plugins: PluginItem[] = [];
-
+  let code = data;
   if (options.instrumentForCoverage) {
-    try {
-      let resolvedPath = 'istanbul-lib-instrument';
-      try {
-        const requireFn = createRequire(filename);
-        resolvedPath = requireFn.resolve('istanbul-lib-instrument');
-      } catch {
-        // Fallback to pool worker import traversal
-      }
-
-      const istanbul = await import(resolvedPath);
-      const programVisitor = istanbul.programVisitor ?? istanbul.default?.programVisitor;
-
-      if (!programVisitor) {
-        throw new Error('programVisitor is not available in istanbul-lib-instrument.');
-      }
-
-      const { default: coveragePluginFactory } =
-        await import('../babel/plugins/add-code-coverage.js');
-      plugins.push(coveragePluginFactory(programVisitor) as unknown as PluginItem);
-    } catch (error) {
-      throw new Error(
-        `The 'istanbul-lib-instrument' package is required for code coverage but was not found. Please install the package.`,
-        { cause: error },
-      );
-    }
+    code = await instrumentCoverage(filename, code, useInputSourcemap);
   }
 
+  const shouldLink = !options.skipLinker && (await requiresLinking(filename, code));
+  const plugins: PluginItem[] = [];
+
   if (shouldLink) {
     // Lazy load the linker plugin only when linking is required
     const linkerPlugin = await createLinkerPlugin(options);
@@ -116,13 +133,13 @@ async function transformWithBabel(
     );
   }
 
-  // If no additional transformations are needed, return the data directly
+  // If no additional transformations are needed, return the code directly
   if (plugins.length === 0) {
     // Strip sourcemaps if they should not be used
-    return useInputSourcemap ? data : removeSourceMappingURL(data);
+    return useInputSourcemap ? code : removeSourceMappingURL(code);
   }
 
-  const result = await transformAsync(data, {
+  const result = await transformAsync(code, {
     filename,
     inputSourceMap: (useInputSourcemap ? undefined : false) as undefined,
     sourceMaps: useInputSourcemap ? 'inline' : false,
@@ -133,7 +150,7 @@ async function transformWithBabel(
     plugins,
   });
 
-  const outputCode = result?.code ?? data;
+  const outputCode = result?.code ?? code;
 
   // Strip sourcemaps if they should not be used.
   // Babel will keep the original comments even if sourcemaps are disabled.