Fix constant elements hoisted out of block (#4419)

When block scoped variables caused the block to be wrapped in a closure, the variable `bindings` remained in parent function scope, which caused the JSX element to be hoisted out of the closure.

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/dont-hoist-block-scoped-variables/actual.js

@@ -0,0 +1,11 @@
+function render(flag) {
+  if (flag) {
+    let bar = "bar";
+
+    [].map(() => bar);
+
+    return <foo bar={bar} />;
+  }
+
+  return null;
+}

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/dont-hoist-block-scoped-variables/expected.js

@@ -0,0 +1,17 @@
+function render(flag) {
+  if (flag) {
+    var _ret = function () {
+      var bar = "bar";
+
+      [].map(() => bar);
+
+      return {
+        v: <foo bar={bar} />
+      };
+    }();
+
+    if (typeof _ret === "object") return _ret.v;
+  }
+
+  return null;
+}

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/dont-hoist-block-scoped-variables/options.json

@@ -0,0 +1,7 @@
+{
+  "plugins": [
+    "syntax-jsx",
+    "transform-es2015-block-scoping",
+    "transform-react-constant-elements"
+  ]
+}