[core] Expand use of flow

[rosskevin]

Annotate more files with flow.

squash+merge

Closes #7271

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• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

[rosskevin]

test:coverage: had this error previous to changes:

  1) <MuiThemeProvider /> server side should be able to extract the styles:
     
  NotFoundError
      at HTMLHeadElementImpl.insertBefore (node_modules/jsdom/lib/jsdom/living/nodes/Node-impl.js:208:17)
      at HTMLHeadElement.insertBefore (node_modules/jsdom/lib/jsdom/living/generated/Node.js:131:45)
      at DomRenderer.attach (node_modules/jss/lib/backends/DomRenderer.js:220:17)
      at StyleSheet.attach (node_modules/jss/lib/StyleSheet.js:64:21)
      at renderNew (node_modules/jss-theme-reactor/styleManager.js:151:47)
      at Object.render (node_modules/jss-theme-reactor/styleManager.js:94:14)
      at Style.render (src/styles/withStyles.js:25:57)
      at node_modules/react-dom/lib/ReactCompositeComponent.js:795:21
      at measureLifeCyclePerf (node_modules/react-dom/lib/ReactCompositeComponent.js:75:12)
      at ReactCompositeComponentWrapper._renderValidatedComponentWithoutOwnerOrContext (node_modules/react-dom/lib/ReactCompositeComponent.js:794:25)
      at ReactCompositeComponentWrapper._renderValidatedComponent (node_modules/react-dom/lib/ReactCompositeComponent.js:821:32)
      at ReactCompositeComponentWrapper.performInitialMount (node_modules/react-dom/lib/ReactCompositeComponent.js:361:30)
      at ReactCompositeComponentWrapper.mountComponent (node_modules/react-dom/lib/ReactCompositeComponent.js:257:21)
      at Object.mountComponent (node_modules/react-dom/lib/ReactReconciler.js:45:35)
      at ReactCompositeComponentWrapper.performInitialMount (node_modules/react-dom/lib/ReactCompositeComponent.js:370:34)
      at ReactCompositeComponentWrapper.mountComponent (node_modules/react-dom/lib/ReactCompositeComponent.js:257:21)
      at Object.mountComponent (node_modules/react-dom/lib/ReactReconciler.js:45:35)
      at node_modules/react-dom/lib/ReactServerRendering.js:45:36
      at ReactServerRenderingTransaction.perform (node_modules/react-dom/lib/Transaction.js:143:20)
      at renderToStringImpl (node_modules/react-dom/lib/ReactServerRendering.js:43:24)
      at renderToString (node_modules/react-dom/lib/ReactServerRendering.js:73:10)
      at Context.<anonymous> (src/styles/MuiThemeProvider.spec.js:36:22)

I filed a new issue, because test:unit succeeds while test:coverage fails.

[oliviertassinari]

We miss the test coverage. Something is wrong here.

[rosskevin]

I'll follow this up in the still open #7271