feat(@angular/build): move istanbul-lib-instrument to optional peer dependency

Move `istanbul-lib-instrument` from dependencies to optional peer dependencies. This helps reduce the transitive dependency count for new projects.

BREAKING CHANGE: `istanbul-lib-instrument` is now an optional peer dependency.
Projects using karma with code coverage enabled will need to ensure that istanbul-lib-instrument is installed. Note: `ng update` will automatically add this dependency during the update process.

Author: clydin

modules/testing/builder/BUILD.bazel

@@ -21,6 +21,7 @@ ts_project(
         # resolvable in the test project.
         ":node_modules/@angular/ssr",
         ":node_modules/browser-sync",
+        ":node_modules/istanbul-lib-instrument",
         ":node_modules/jsdom",
         ":node_modules/ng-packagr",
         ":node_modules/vitest",

modules/testing/builder/package.json

@@ -6,6 +6,7 @@
     "@angular/ssr": "workspace:*",
     "@vitest/coverage-v8": "4.1.2",
     "browser-sync": "3.0.4",
+    "istanbul-lib-instrument": "6.0.3",
     "jsdom": "29.0.1",
     "ng-packagr": "22.0.0-next.1",
     "rxjs": "7.8.2",

packages/angular/build/package.json

@@ -29,7 +29,6 @@
     "browserslist": "^4.26.0",
     "esbuild": "0.27.3",
     "https-proxy-agent": "8.0.0",
-    "istanbul-lib-instrument": "6.0.3",
     "jsonc-parser": "3.3.1",
     "listr2": "10.2.1",
     "magic-string": "0.30.21",
@@ -51,6 +50,7 @@
   "devDependencies": {
     "@angular-devkit/core": "workspace:*",
     "@angular/ssr": "workspace:*",
+    "istanbul-lib-instrument": "6.0.3",
     "jsdom": "29.0.1",
     "less": "4.6.4",
     "ng-packagr": "22.0.0-next.1",
@@ -67,6 +67,7 @@
     "@angular/platform-server": "0.0.0-ANGULAR-FW-PEER-DEP",
     "@angular/service-worker": "0.0.0-ANGULAR-FW-PEER-DEP",
     "@angular/ssr": "^0.0.0-PLACEHOLDER",
+    "istanbul-lib-instrument": "^6.0.0",
     "karma": "^6.4.0",
     "less": "^4.2.0",
     "ng-packagr": "0.0.0-NG-PACKAGR-PEER-DEP",
@@ -95,6 +96,9 @@
     "@angular/ssr": {
       "optional": true
     },
+    "istanbul-lib-instrument": {
+      "optional": true
+    },
     "karma": {
       "optional": true
     },

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

@@ -7,15 +7,17 @@
  */
 
 import { NodePath, PluginObj, types } from '@babel/core';
-import { Visitor, programVisitor } from 'istanbul-lib-instrument';
+import type { Visitor } from 'istanbul-lib-instrument';
 import assert from 'node:assert';
 
 /**
  * A babel plugin factory function for adding istanbul instrumentation.
  *
  * @returns A babel plugin object instance.
  */
-export default function (): PluginObj {
+export default function (
+  programVisitor: typeof import('istanbul-lib-instrument').programVisitor,
+): PluginObj {
   const visitors = new WeakMap<NodePath, Visitor>();
 
   return {

packages/angular/build/src/tools/esbuild/javascript-transformer-worker.ts

@@ -8,6 +8,7 @@
 
 import { type PluginItem, transformAsync } from '@babel/core';
 import fs from 'node:fs';
+import { createRequire } from 'node:module';
 import path from 'node:path';
 import Piscina from 'piscina';
 
@@ -66,8 +67,32 @@ async function transformWithBabel(
   const plugins: PluginItem[] = [];
 
   if (options.instrumentForCoverage) {
-    const { default: coveragePlugin } = await import('../babel/plugins/add-code-coverage.js');
-    plugins.push(coveragePlugin);
+    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));
+    } catch (error) {
+      const details = error instanceof Error ? error.message : String(error);
+      throw new Error(
+        `The 'istanbul-lib-instrument' package is required for code coverage but was not found or failed to load. Details: ${details}`,
+        { cause: error },
+      );
+    }
   }
 
   if (shouldLink) {