Fix running flow on travis and update flow (#9128)

* Fix running flow on travis and update flow

- ensure bootstrap is run before running flow as we need some generated files for correctly doing typechecks
- ensure that we only ignore the build directory inside the babel folder as currently we ignore everything because travis checks out into ‘/home/travis/build/’

* Fix all flow errors

packages/babel-core/src/config/caching.js

@@ -43,6 +43,7 @@ export function makeWeakCache<
 >(
   handler: (ArgT, CacheConfigurator<SideChannel>) => ResultT,
 ): (ArgT, SideChannel) => ResultT {
+  // $FlowIssue https://github.com/facebook/flow/issues/4528
   return makeCachedFunction(new WeakMap(), handler);
 }
 
@@ -54,6 +55,7 @@ function makeCachedFunction<
   ArgT,
   ResultT,
   SideChannel,
+  // $FlowIssue https://github.com/facebook/flow/issues/4528
   Cache: CacheMap<ArgT, ResultT, SideChannel>,
 >(
   callCache: Cache,

packages/babel-core/src/config/validation/options.js

@@ -350,7 +350,7 @@ function validateNested(loc: NestingPath, opts: {}) {
       NONPRESET_VALIDATORS[key] ||
       BABELRC_VALIDATORS[key] ||
       ROOT_VALIDATORS[key] ||
-      throwUnknownError;
+      (throwUnknownError: Validator<void>);
 
     validator(optLoc, opts[key]);
   });