Split exportExtensions into exportDefault and exportNamespace plugins… (#6920)

* Split exportExtensions into two plugins in babylon
* rename proposal-export-default to proposal-export-default-from
* rename proposal-export-namespace to proposal-export-namespace-from

packages/babel-plugin-proposal-export-default-from/src/index.js

@@ -0,0 +1,31 @@
+import syntaxExportDefaultFrom from "@babel/plugin-syntax-export-default-from";
+import { types as t } from "@babel/core";
+
+export default function() {
+  return {
+    inherits: syntaxExportDefaultFrom,
+
+    visitor: {
+      ExportNamedDeclaration(path) {
+        const { node, scope } = path;
+        const { specifiers } = node;
+        if (!t.isExportDefaultSpecifier(specifiers[0])) return;
+
+        const specifier = specifiers.shift();
+        const { exported } = specifier;
+        const uid = scope.generateUidIdentifier(exported.name);
+
+        const nodes = [
+          t.importDeclaration([t.importDefaultSpecifier(uid)], node.source),
+          t.exportNamedDeclaration(null, [t.exportSpecifier(uid, exported)]),
+        ];
+
+        if (specifiers.length >= 1) {
+          nodes.push(node);
+        }
+
+        path.replaceWithMultiple(nodes);
+      },
+    },
+  };
+}