fix(@schematics/angular): only remove tslib from package.json when it's not a dependency

Fixes: #16191

Author: alan-agius4

packages/schematics/angular/migrations/update-9/add-tslib.ts

@@ -6,16 +6,22 @@
  * found in the LICENSE file at https://angular.io/license
  */
 import { Rule } from '@angular-devkit/schematics';
-import { NodeDependencyType, addPackageJsonDependency, removePackageJsonDependency } from '../../utility/dependencies';
+import { NodeDependencyType, addPackageJsonDependency, getPackageJsonDependency, removePackageJsonDependency } from '../../utility/dependencies';
 import { latestVersions } from '../../utility/latest-versions';
 
 export function addTsLib(): Rule {
   return host => {
-    removePackageJsonDependency(host, 'tslib');
+    const tslibDep = getPackageJsonDependency(host, 'tslib');
+
+    if (tslibDep && tslibDep.type !== NodeDependencyType.Default) {
+      removePackageJsonDependency(host, 'tslib');
+    }
+
     addPackageJsonDependency(host, {
-        name: 'tslib',
-        version: latestVersions.TsLib,
-        type: NodeDependencyType.Default,
+      name: 'tslib',
+      version: latestVersions.TsLib,
+      type: NodeDependencyType.Default,
+      overwrite: true,
     });
   };
 }