Transform do-expressions on exit (babel#5694)

* Transform do-expressions on exit

This allows other transforms (notably, block scoping) to do their magic
first, possibly saving closures.

Also fixes a bug with declaring duplicate bindings (de-opts now).

* Use strict in exec test

* lint

packages/babel-plugin-transform-do-expressions/src/index.js

@@ -5,13 +5,15 @@ export default function () {
     inherits: syntaxDoExpressions,
 
     visitor: {
-      DoExpression(path) {
-        const body = path.node.body.body;
-        if (body.length) {
-          path.replaceWithMultiple(body);
-        } else {
-          path.replaceWith(path.scope.buildUndefinedNode());
-        }
+      DoExpression: {
+        exit(path) {
+          const body = path.node.body.body;
+          if (body.length) {
+            path.replaceExpressionWithStatements(body);
+          } else {
+            path.replaceWith(path.scope.buildUndefinedNode());
+          }
+        },
       },
     },
   };

packages/babel-plugin-transform-do-expressions/test/fixtures/do-expressions/block-scope.js

@@ -0,0 +1,10 @@
+"use strict";
+
+let a = 1;
+
+(do {
+  let a = 2;
+  assert.equal(a, 2);
+});
+assert.equal(a, 1);
+

packages/babel-plugin-transform-do-expressions/test/fixtures/do-expressions/block-scope/actual.js

@@ -0,0 +1,7 @@
+let a = 1;
+
+(do {
+  let a = 2;
+  a;
+});
+

packages/babel-plugin-transform-do-expressions/test/fixtures/do-expressions/block-scope/expected.js

@@ -0,0 +1,5 @@
+var _a;
+
+var a = 1;
+
+_a = 2, _a;

packages/babel-plugin-transform-do-expressions/test/fixtures/do-expressions/block-scope/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["transform-es2015-block-scoping", "transform-do-expressions"]
+}