Prevent nameclashes with internal variables

Fixes #1699

src/scope-hoisting/hoist.js

@@ -121,7 +121,7 @@ module.exports = {
         // Rename each binding in the top-level scope to something unique.
         for (let name in scope.bindings) {
           if (!name.startsWith('$' + t.toIdentifier(asset.id))) {
-            let newName = getName(asset, 'var', name);
+            let newName = getName(asset, 'var', name, '$');
             rename(scope, name, newName);
           }
         }

test/integration/scope-hoisting/es6/name-clash/a.js

@@ -0,0 +1,3 @@
+import {foo} from './b'
+
+output = foo()

test/integration/scope-hoisting/es6/name-clash/b.js

@@ -0,0 +1,3 @@
+var $exports = module.exports = {}
+
+$exports.foo = () => 'bar'

test/scope-hoisting.js

@@ -330,6 +330,15 @@ describe('scope hoisting', function() {
       let output = await run(b);
       assert.deepEqual(output, 'bar');
     });
+
+    it('should not nameclash with internal variables', async function() {
+      let b = await bundle(
+        __dirname + '/integration/scope-hoisting/es6/name-clash/a.js'
+      );
+
+      let output = await run(b);
+      assert.deepEqual(output, 'bar');
+    });
   });
 
   describe('commonjs', function() {