ng test: Collect total code coverage in multi project repository

先使用 ng test 重現多專案 workspace 的情況，然後檢查 application 和 library schematics 下所參照的 karma.conf.js 檔案。追蹤 coverage 的處理應歸屬於 builder 還是 schematic；完成的條件是每個專案都有自己的 lcov 資料，且合併報告能夠呈現整體 coverage。

Bug Report or Feature Request (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x] feature request

Area

- [x] devkit
- [ ] schematics

Versions

Angular CLI: 6.0.8
Node: 8.9.1
OS: win32 x64
Angular: 5.2.11
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.6.8
@angular-devkit/build-angular     0.6.8
@angular-devkit/build-optimizer   0.6.8
@angular-devkit/core              0.6.8
@angular-devkit/schematics        0.6.8
@angular/cdk                      5.2.5
@angular/cli                      6.0.8
@ngtools/json-schema              1.1.0
@ngtools/webpack                  6.0.8
@schematics/angular               0.6.1
@schematics/update                0.6.8
ng-packagr                        3.0.2
rxjs                              6.2.1
typescript                        2.6.2
webpack                           4.8.3

Repro steps

Multi project workspace, run ng test to test all projects.

The log given by the failure

Produced coverage report in coverage/lcov.info only contains results for the last run code coverage.

HTML reports are stored per project in coverage/<project>.

Desired functionality

Write indivudual lcovs per project, e.g, coverage/<project>/lcov.info, allowing users to get a total code coverage for the whole project.

Alternative would be functionality similar to istanbul-combine. Write coverage.json for each project and write a combined coverage report and end of test runs.

Mention any other details that might be useful

Should eht changes be made in the builder or in the schematic?

https://github.com/angular/angular-cli/blob/master/packages/schematics/angular/library/files/__projectRoot__/karma.conf.js#L19

https://github.com/angular/angular-cli/blob/master/packages/schematics/angular/application/files/root/karma.conf.js#L19