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/src/index.js

@@ -0,0 +1,56 @@
+import syntaxFunctionSent from "babel-plugin-syntax-function-sent";
+import wrapFunction from "babel-helper-wrap-function";
+
+export default function({ types: t }) {
+  const isFunctionSent = node =>
+    t.isIdentifier(node.meta, { name: "function" }) &&
+    t.isIdentifier(node.property, { name: "sent" });
+
+  const yieldVisitor = {
+    Function(path) {
+      path.skip();
+    },
+
+    YieldExpression(path) {
+      const replaced = t.isAssignmentExpression(path.parent, {
+        left: this.sentId,
+      });
+      if (!replaced) {
+        path.replaceWith(t.assignmentExpression("=", this.sentId, path.node));
+      }
+    },
+
+    MetaProperty(path) {
+      if (isFunctionSent(path.node)) {
+        path.replaceWith(this.sentId);
+      }
+    },
+  };
+
+  return {
+    inherits: syntaxFunctionSent,
+
+    visitor: {
+      MetaProperty(path, state) {
+        if (!isFunctionSent(path.node)) return;
+
+        const fnPath = path.getFunctionParent();
+
+        if (!fnPath.node.generator) {
+          throw new Error("Parent generator function not found");
+        }
+
+        const sentId = path.scope.generateUidIdentifier("function.sent");
+
+        fnPath.traverse(yieldVisitor, { sentId });
+        fnPath.node.body.body.unshift(
+          t.variableDeclaration("let", [
+            t.variableDeclarator(sentId, t.yieldExpression()),
+          ]),
+        );
+
+        wrapFunction(fnPath, state.addHelper("skipFirstGeneratorNext"));
+      },
+    },
+  };
+}