ng test: Collect total code coverage in multi project repository

首先使用 ng test 重现多项目 workspace 的情况，然后检查 application 和 library schematics 下引用的 karma.conf.js 文件。跟踪 coverage 的处理应归属于 builder 还是 schematic；完成的标准是每个项目都有自己的 lcov 数据，并且组合报告能够表示总体 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