add do expressions

src/acorn/src/expression.js

@@ -245,6 +245,14 @@ pp.parseExprAtom = function(refShorthandDefaultPos) {
   case tt._yield:
     if (this.inGenerator) unexpected()
 
+  case tt._do:
+    if (this.options.features["es7.doExpressions"]) {
+      let node = this.startNode()
+      this.next()
+      node.body = this.parseBlock()
+      return this.finishNode(node, "DoExpression")
+    }
+
   case tt.name:
     let start = this.markPosition()
     node = this.startNode()

src/babel/transformation/transformers/es7/do-expressions.js

@@ -0,0 +1,15 @@
+import * as t from "../../../types";
+
+export var metadata = {
+  experimental: true,
+  optional: true
+};
+
+export function DoExpression(node) {
+  var body = node.body.body;
+  if (body.length) {
+    return body;
+  } else {
+    return t.identifier("undefined");
+  }
+}

src/babel/transformation/transformers/index.js

@@ -90,6 +90,8 @@ export default {
 
   _shadowFunctions:                        require("./internal/alias-functions"),
 
+  "es7.doExpressions":                     require("./es7/do-expressions"),
+
   "es6.symbols":                           require("./es6/symbols"),
   "spec.undefinedToVoid":                  require("./spec/undefined-to-void"),
 

src/babel/types/alias-keys.json

@@ -57,6 +57,7 @@
   "CallExpression":            ["Expression"],
   "ComprehensionExpression":   ["Expression", "Scopable"],
   "ConditionalExpression":     ["Expression"],
+  "DoExpression":              ["Expression"],
   "Identifier":                ["Expression"],
   "Literal":                   ["Expression"],
   "MemberExpression":          ["Expression"],

src/babel/types/visitor-keys.json

@@ -19,6 +19,7 @@
   "ContinueStatement":         ["label"],
   "DebuggerStatement":         [],
   "DoWhileStatement":          ["body", "test"],
+  "DoExpression":              ["body"],
   "EmptyStatement":            [],
   "ExportAllDeclaration":      ["source"],
   "ExportDefaultDeclaration":  ["declaration"],