Function sent (#5920)

* Create "babel-helper-wrap-function"

It contains the logic to wrap a function inside a call expression.
It was part of the "babel-helper-remap-async-to-generator" package, but
it is needed to transpile "function.sent"

* Create "babel-transform-function-sent"

It transforms the "function.sent" meta property by replacing it with
"yield" and making the generator ignore the first ".next()" call.

* "function.sent" is the last value passed to .next(), not the first one

* Disable exec tests on old node

* Fix flow error

* Add "transform-function-sent" to "stage-2" preset

* Do every trasformation in one traversal

* Test for "yield function.sent"

* [skip ci]

* Fix some typos [skip ci]

packages/babel-plugin-transform-function-sent/test/fixtures/function-sent/yield-function-sent/actual.js

@@ -0,0 +1,3 @@
+(function* () {
+  yield function.sent;
+})();

packages/babel-plugin-transform-function-sent/test/fixtures/function-sent/yield-function-sent/expected.js

@@ -0,0 +1,7 @@
+function _skipFirstGeneratorNext(fn) { return function () { var it = fn.apply(this, arguments); it.next(); return it; }; }
+
+_skipFirstGeneratorNext(function* () {
+  let _functionSent = yield;
+
+  _functionSent = yield _functionSent;
+})();