Add test for flow type export

[hazeledmands]

This adds a test for the bug reported in issue #758 and #660, which was already fixed by bfdc2bb. :)

If I revert that commit, the test fails with this error:

  1) no-mutable-exports valid type Foo = {}
export type {Foo}:
     TypeError: Cannot read property 'kind' of null
      at checkDeclaration (src/rules/no-mutable-exports.js:7:37)
      at checkDeclarationsInScope (src/rules/no-mutable-exports.js:19:15)
      at EventEmitter.handleExportNamed (src/rules/no-mutable-exports.js:41:11)
      at NodeEventGenerator.applySelector (node_modules/eslint/lib/util/node-event-generator.js:265:26)
      at NodeEventGenerator.applySelectors (node_modules/eslint/lib/util/node-event-generator.js:294:22)
      at NodeEventGenerator.enterNode (node_modules/eslint/lib/util/node-event-generator.js:308:14)
      at CodePathAnalyzer.enterNode (node_modules/eslint/lib/code-path-analysis/code-path-analyzer.js:602:23)
      at CommentEventGenerator.enterNode (node_modules/eslint/lib/util/comment-event-generator.js:98:23)
      at Traverser.enter (node_modules/eslint/lib/eslint.js:929:36)
      at Traverser.__execute (node_modules/estraverse/estraverse.js:397:31)
      at Traverser.traverse (node_modules/estraverse/estraverse.js:501:28)
      at Traverser.traverse (node_modules/eslint/lib/util/traverser.js:31:22)
      at EventEmitter.module.exports.api.verify (node_modules/eslint/lib/eslint.js:926:23)
      at runRuleForItem (node_modules/eslint/lib/testers/rule-tester.js:387:38)
      at testValidTemplate (node_modules/eslint/lib/testers/rule-tester.js:420:28)
      at Context.RuleTester.it (node_modules/eslint/lib/testers/rule-tester.js:548:25)

[coveralls]

Coverage remained the same at 94.915% when pulling b4dcd43 on demands:support-flow into bd0e5e3 on benmosher:master.