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/multiple/actual.js

@@ -0,0 +1,10 @@
+(function* () {
+  const a = function.sent;
+  const b = function.sent;
+  yield 4;
+  const c = function.sent;
+  const d = yield;
+  const e = function.sent;
+
+  return [ a, b, c, d, e ];
+}());

packages/babel-plugin-transform-function-sent/test/fixtures/function-sent/multiple/exec.js

@@ -0,0 +1,23 @@
+const values = [];
+
+function* gen() {
+  values.push(function.sent);
+  values.push(function.sent);
+  values.push(yield "foo");
+  values.push(function.sent);
+  values.push(yield);
+  values.push(function.sent);
+  values.push(function.sent);
+}
+
+const it = gen();
+assert.deepEqual(values, []);
+
+assert.equal(it.next(1).value, "foo");
+assert.deepEqual(values, [ 1, 1 ]);
+
+assert.equal(it.next(2).value, undefined);
+assert.deepEqual(values, [ 1, 1, 2, 2 ]);
+
+assert.equal(it.next(3).done, true);
+assert.deepEqual(values, [ 1, 1, 2, 2, 3, 3, 3 ]);

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

@@ -0,0 +1,13 @@
+function _skipFirstGeneratorNext(fn) { return function () { var it = fn.apply(this, arguments); it.next(); return it; }; }
+
+_skipFirstGeneratorNext(function* () {
+  let _functionSent = yield;
+
+  const a = _functionSent;
+  const b = _functionSent;
+  _functionSent = yield 4;
+  const c = _functionSent;
+  const d = _functionSent = yield;
+  const e = _functionSent;
+  return [a, b, c, d, e];
+})();

packages/babel-plugin-transform-function-sent/test/fixtures/function-sent/multiple/options.json

@@ -0,0 +1,3 @@
+{
+  "minNodeVersion": "6.0.0"
+}