Fix parsing of private fields (#566)

The computed key is not part of the spec.
key for ClassProperties is an Expression
Do not parse computed and literal keys for PrivateClassProperties

src/plugins/flow.js

@@ -1294,7 +1294,6 @@ export default (superClass: Class<Parser>): Class<Parser> => class extends super
   parsePropertyName(node: N.ObjectOrClassMember): N.Identifier {
     const variance = this.flowParseVariance();
     const key = super.parsePropertyName(node);
-    // $FlowFixMe (variance not defined on ClassPrivateProperty)
     node.variance = variance;
     return key;
   }