Add throw expressions (#6325)

* Add throw expressions

Stage 2 proposal: https://github.com/tc39/proposal-throw-expressions

* Update babylon

* Add to stage 2

packages/babel-types/src/constants.js

@@ -47,6 +47,7 @@ export const NUMBER_UNARY_OPERATORS = ["+", "-", "~"];
 export const STRING_UNARY_OPERATORS = ["typeof"];
 export const UNARY_OPERATORS = [
   "void",
+  "throw",
   ...BOOLEAN_UNARY_OPERATORS,
   ...NUMBER_UNARY_OPERATORS,
   ...STRING_UNARY_OPERATORS,