fix(@angular-devkit/build-angular): experimental esbuild pipeline, ad…

…d `es2015` to main fields for RxJS v6 compatibility

Currently, for RxJS v6, no conditions are available, and the ESBuild
pipeline will select the ES5 distribution based on the `module` main
field. This is fine in most cases, but applications could benefit from
better optimization with the use of the ES2015 output + there are
certain code differences that currently would cause runtime breakages
when e.g. `rxjs/testing` is used.

See: #25405 for more details.

Fixes #25405.

(cherry picked from commit 9d9a668)

packages/angular_devkit/build_angular/src/tools/esbuild/application-code-bundle.ts

@@ -47,7 +47,11 @@ export function createCodeBundleOptions(
     assetNames: outputNames.media,
     target,
     supported: getFeatureSupport(target),
-    mainFields: ['es2020', 'browser', 'module', 'main'],
+    // Note: `es2015` is needed for RxJS v6. If not specified, `module` would
+    // match and the ES5 distribution would be bundled and ends up breaking at
+    // runtime with the RxJS testing library.
+    // More details: https://github.com/angular/angular-cli/issues/25405.
+    mainFields: ['es2020', 'es2015', 'browser', 'module', 'main'],
     conditions: ['es2020', 'es2015', 'module'],
     resolveExtensions: ['.ts', '.tsx', '.mjs', '.js'],
     metafile: true,