fix(umi-build-dev): config change don't update service's config

[sorrycc]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 575

• 0 of 5 (0.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 21.737%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/umi-build-dev/src/UserConfig.js	0	2	0.0%
packages/umi-build-dev/src/Service.js	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
packages/umi-build-dev/src/Service.js	1	0.0%

Totals
Change from base Build 571:	-0.03%
Covered Lines:	519
Relevant Lines:	2618

---

💛 - Coveralls

[sorrycc]

补充下 testcase:

plugin

import assert from 'assert';

export default (api) => {
  const { config } = api.service;
  assert(config.singular === undefined, 'modifyConfig 修改的配置在方法外不生效');

  let modified = false;

  api.register('modifyConfig', ({ memo }) => {
    return {
      ...memo,
      singular: true,
    };
  });

  api.register('onStart', () => {
    assert(config.ctoken === 1);
    assert(api.service.config.ctoken === 1);
    assert(config.singular === true);
    assert(api.service.config.singular === true);
  });

  api.register('modifyConfigPlugins', ({ memo }) => {
    memo.push(api => ({
      name: 'ctoken',
      onChange(c) {
        assert(c.ctoken === 2);
        assert(api.service.config.ctoken === 2);
        modified = true;
        api.service.filesGenerator.rebuild();
      },
    }));
    return memo;
  });

  api.register('modifyEntryFile', ({ memo }) => {
    if (modified) {
      assert(config.ctoken === 2);
      assert(api.service.config.ctoken === 2);
    } else {
      assert(config.ctoken === 1);
      assert(api.service.config.ctoken === 1);
    }
    assert(config.singular === true);
    assert(api.service.config.singular === true);
    return memo;
  })
}

.umirc.js

export default {
  plugins: [
    './plugin',
  ],
  ctoken: 1,
};

启动后修改 ctoken 为 2。