fix(@angular/cli): set correct mainFields for ng test when targeting es2015

Added test fails without the fix with the error described in the linked issue.

Fixes #8379

Author: devoto13

packages/@angular/cli/models/webpack-configs/test.ts

@@ -48,6 +48,12 @@ export function getTestConfig(wco: WebpackConfigOptions<WebpackTestOptions>) {
   }
 
   return {
+    resolve: {
+      mainFields: [
+        ...(wco.supportES2015 ? ['es2015'] : []),
+        'browser', 'module', 'main'
+      ]
+    },
     devtool: buildOptions.sourcemaps ? 'inline-source-map' : 'eval',
     entry: {
       main: path.resolve(projectRoot, appConfig.root, appConfig.test)

tests/e2e/tests/test/test-target.ts

@@ -0,0 +1,14 @@
+import { ng } from '../../utils/process';
+import { updateJsonFile } from '../../utils/project';
+
+export default function () {
+  return updateJsonFile('tsconfig.json', configJson => {
+    const compilerOptions = configJson['compilerOptions'];
+    compilerOptions['target'] = 'es2015';
+  })
+    .then(() => updateJsonFile('src/tsconfig.spec.json', configJson => {
+      const compilerOptions = configJson['compilerOptions'];
+      compilerOptions['target'] = 'es2015';
+    }))
+    .then(() => ng('test', '--single-run'));
+}