fix const name collisions

with the `always: true` option, code like
```js
const Buffer = null
```
could cause parse errors because we generated:
```js
(function(Buffer){
    const Buffer = null
}())
```
which is a disallowed redeclaration.

This change adds an extra layer of IIFE wrapping so that the
`const Buffer` _shadows_ the parameter name, instead of redeclaring it.

Author: goto-bus-stop

index.js

@@ -154,10 +154,13 @@ function closeOver (globals, src, file, opts) {
     if (keys.length === 0) return src;
     var values = keys.map(function (key) { return globals[key] });
 
-    var wrappedSource;
+    // we double-wrap the source in IIFEs to prevent code like
+    //     (function(Buffer){ const Buffer = null }())
+    // which causes a parse error.
+    var wrappedSource = '(function (){\n' + src + '\n}).call(this)';
     if (keys.length <= 3) {
-        wrappedSource = '(function (' + keys.join(',') + '){\n'
-            + src + '\n}).call(this,' + values.join(',') + ')'
+        wrappedSource = '(function (' + keys.join(',') + '){'
+            + wrappedSource + '}).call(this,' + values.join(',') + ')'
         ;
     }
     else {
@@ -169,8 +172,8 @@ function closeOver (globals, src, file, opts) {
           'arguments[3]','arguments[4]',
           'arguments[5]','arguments[6]'
       );
-      wrappedSource = '(function (' + names.join(',') + '){\n'
-        + src + '\n}).call(this,' + values.join(',') + ')';
+      wrappedSource = '(function (' + names.join(',') + '){'
+        + wrappedSource + '}).call(this,' + values.join(',') + ')';
     }
 
     // Generate source maps if wanted. Including the right offset for

test/always.js

@@ -101,6 +101,34 @@ test('always truthy-but-not-true insert hidden from quick test is not really ins
     testit(always, expected);
 });
 
+test('inserted names do not cause const name collisions', function (t) {
+    t.plan(1);
+    var s = mdeps({
+        modules: {
+            buffer: require.resolve('buffer/'),
+            timers: require.resolve('timers-browserify')
+        }
+    });
+    s.on('error', t.fail);
+    s.pipe(bpack({ raw: true })).pipe(concat(function (src) {
+        var c = {
+            t: t,
+            Buffer: 'sandbox Buffer'
+        };
+        vm.runInNewContext(src, c);
+    }));
+    s.write({
+        transform: inserter({
+            always: true,
+            vars: {
+                Buffer: function() { return '"sandbox Buffer"' }
+            }
+        }),
+        global: true
+    });
+    s.end(__dirname + '/always/collision.js');
+});
+
 function inserter (opts) {
     return function (file) {
         return insert(file, opts);

test/always/collision.js

@@ -0,0 +1,2 @@
+const Buffer = null;
+t.equal(Buffer, null);