fix(prettier): overwrite existing .prettierrc

- workaorund introduce due angular/angular-cli#11337

src/prettier/index.spec.ts

@@ -135,12 +135,13 @@ describe('When prettier is already configured', () => {
     project.create('package.json', JSON.stringify(packageBeforeInstall));
   });
 
-  it(' should warn if configuration is detected in package.json', () => {
+  it('should warn if configuration is detected in package.json', () => {
     runner.runSchematic('prettier', {}, project);
     expect(warn).toHaveBeenCalledWith(
       'Found competing prettier configuration in package.json.'
     );
   });
+
   it.each([
     ['.prettierrc.yaml'],
     ['.prettierrc.yml'],

src/prettier/rules/prettier/configure-prettier.ts

@@ -1,18 +1,31 @@
 import {
   apply,
   chain,
+  forEach,
   MergeStrategy,
   mergeWith,
   Rule,
+  Tree,
   url
 } from '@angular-devkit/schematics';
 import { installDependencies } from '../../../lib';
 
 export function configurePrettier(): Rule {
-  return chain([
-    installDependencies({
-      devDependencies: ['prettier', 'tslint-config-prettier']
-    }),
-    mergeWith(apply(url('./templates/prettier'), []), MergeStrategy.Overwrite)
-  ]);
+  return (tree: Tree) =>
+    chain([
+      installDependencies({
+        devDependencies: ['prettier', 'tslint-config-prettier']
+      }),
+      mergeWith(
+        apply(url('./templates/prettier'), [
+          forEach(template => {
+            tree.exists(template.path)
+              ? tree.overwrite(template.path, template.content)
+              : tree.create(template.path, template.content);
+            return null;
+          })
+        ]),
+        MergeStrategy.Overwrite
+      )
+    ]);
 }