keywords are not allowed as local specifier in imports

[danez]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	yes
Tests added/pass?	yes
Fixed tickets
License	MIT

This should not be allowed

import { default } from "foo";

https://www.ecma-international.org/ecma-262/7.0/#prod-BindingIdentifier

The Identifier is an BindingIdentifier and the spec says not ReservedKeyword

As flow completely overwrites the parsing I also had to change it there.
Furthermore I fixed the implementation in the flow plugin to consider keywords and not only names and also changed parsing Identifier to always do the ReservedKeyword check, so that the error message gets clearer.
Before: Unexpected token (what else would it be?)
After : <keyword> is a reserved word

[codecov-io]

Current coverage is 97.19% (diff: 100%)

Merging #307 into master will increase coverage by 0.06%

@@             master       #307   diff @@
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  Files            21         21          
  Lines          3487       3492     +5   
  Methods         400        400          
  Messages          0          0          
  Branches        896        895     -1   
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+ Hits           3387       3394     +7   
+ Misses           44         43     -1   
+ Partials         56         55     -1   

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