Spec compatibility for iteratorClose condition.

[yavorsky]

Q	A
Fixed Issues
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added/Pass?	👍
Spec Compliancy?	👍
License	MIT
Doc PR
Any Dependency Changes?

Currently, we are using just ITERATOR_KEY.return in the condition before we are calling iterator.return(), but according to the spec we need to check it for abrupt completion:
!_iteratorNormalCompletion && _iterator.return != null.

Spec ref:
Step 3 - https://tc39.github.io/ecma262/#sec-iteratorclose

Repl:
https://babeljs.io/repl/#?babili=false&evaluate=true&lineWrap=false&presets=es2015&targets=Node-7.3&browsers=ie%2010&builtIns=false&debug=false&code_lz=GYewTgBAFAxiB2BnALhAhhEwIFs0AcBKCAbwF8g&experimental=true&loose=false&spec=false&playground=true&stage=0

[mention-bot]

@yavorsky, thanks for your PR! By analyzing the history of the files in this pull request, we identified @zenparsing, @hzoo and @existentialism to be potential reviewers.

[loganfsmyth]

There's also

babel/packages/babel-helpers/src/helpers.js

Line 531 in a04c18a

if (!_n && _i["return"]) _i["return"]();