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

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",
@@ -67,6 +66,7 @@
     "@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 +95,9 @@
     "@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

@@ -66,8 +66,18 @@ async function transformWithBabel(
   const plugins: PluginItem[] = [];
 
   if (options.instrumentForCoverage) {
-    const { default: coveragePlugin } = await import('../babel/plugins/add-code-coverage.js');
-    plugins.push(coveragePlugin);
+    try {
+      const { programVisitor } = await import('istanbul-lib-instrument');
+      const { default: coveragePluginFactory } = await import(
+        '../babel/plugins/add-code-coverage.js'
+      );
+      plugins.push(coveragePluginFactory(programVisitor));
+    } catch {
+      throw new Error(
+        "The 'istanbul-lib-instrument' package is required for code coverage but was not found. " +
+          'Please install it.',
+      );
+    }
   }
 
   if (shouldLink) {