test(@angular-devkit/build-angular): improve performance of autoprefixer tests

These tests check the output of the global stylesheet and require a browserslist to target IE but this also triggers differential loading which is unneeded for the test.

Author: clydin

packages/angular_devkit/build_angular/src/browser/specs/styles_spec.ts

@@ -305,6 +305,9 @@ describe('Browser Builder styles', () => {
       '.browserslistrc': 'IE 10',
     });
 
+    // Set to target to ES5 to avoid differential loading and unnecessary testing time
+    host.replaceInFile('tsconfig.json', '"target": "es2015"', '"target": "es5"');
+
     const overrides = { extractCss: true, optimization: false };
     const { files } = await browserBuild(architect, host, target, overrides);
     expect(await files['styles.css']).toContain(tags.stripIndents`
@@ -342,6 +345,9 @@ describe('Browser Builder styles', () => {
       '.browserslistrc': 'IE 10',
     });
 
+    // Set target to ES5 to avoid differential loading and unnecessary testing time
+    host.replaceInFile('tsconfig.json', '"target": "es2015"', '"target": "es5"');
+
     const overrides = { extractCss: true, optimization: true, styles: ['src/styles.scss'] };
     const { files } = await browserBuild(architect, host, target, overrides);
     expect(await files['styles.css']).toContain('-ms-grid-columns:100px;');