fix(@schematics/angular): migrate existing packages for 7.0 (#12518)

Fixes: #12458

Author: clydin

packages/schematics/angular/migrations/update-7/index.ts

@@ -10,17 +10,21 @@ import { NodePackageInstallTask } from '@angular-devkit/schematics/tasks';
 import {
   NodeDependencyType,
   addPackageJsonDependency,
+  getPackageJsonDependency,
 } from '../../utility/dependencies';
 import { latestVersions } from '../../utility/latest-versions';
 
 export { polyfillMetadataRule } from './polyfill-metadata';
 
 export default function(): Rule {
   return (tree, context) => {
+    const existing = getPackageJsonDependency(tree, '@angular-devkit/build-angular');
+    const type = existing ? existing.type : NodeDependencyType.Dev;
+
     addPackageJsonDependency(
       tree,
       {
-        type: NodeDependencyType.Dev,
+        type,
         name: '@angular-devkit/build-angular',
         version: latestVersions.DevkitBuildAngular,
         overwrite: true,