Fixes #147—Ensures core modules beat shadowed ones

Given 'util' as an example native module which is also installed as an
npm package, then in order for browserify/resolve to act consistently
with native `require.resolve`, these should be true:

```
resolve.sync('util') // returns 'util'
resolve.sync('util/') // returns 'node_modules/util/index.js'
```

lib/async.js

@@ -41,10 +41,13 @@ module.exports = function resolve(x, options, callback) {
             loadAsDirectory(res, opts.package, onfile);
         } else loadAsFile(res, opts.package, onfile);
     } else loadNodeModules(x, y, function (err, n, pkg) {
-        if (err) cb(err);
-        else if (n) cb(null, n, pkg);
-        else if (core[x]) return cb(null, x);
-        else {
+        if (err) {
+            cb(err);
+        } else if (core[x]) {
+            cb(null, x);
+        } else if (n) {
+            cb(null, n, pkg);
+        } else {
             var moduleError = new Error("Cannot find module '" + x + "' from '" + y + "'");
             moduleError.code = 'MODULE_NOT_FOUND';
             cb(moduleError);

lib/sync.js

@@ -30,13 +30,13 @@ module.exports = function (x, options) {
         if (x === '..' || x.slice(-1) === '/') res += '/';
         var m = loadAsFileSync(res) || loadAsDirectorySync(res);
         if (m) return m;
+    } else if (core[x]) {
+        return x;
     } else {
         var n = loadNodeModulesSync(x, y);
         if (n) return n;
     }
 
-    if (core[x]) return x;
-
     var err = new Error("Cannot find module '" + x + "' from '" + y + "'");
     err.code = 'MODULE_NOT_FOUND';
     throw err;

test/shadowed_core.js

@@ -11,6 +11,14 @@ test('shadowed core modules still return core module', function (t) {
     });
 });
 
+test('shadowed core modules still return core module [sync]', function (t) {
+    t.plan(1);
+
+    var res = resolve.sync('util', { basedir: path.join(__dirname, 'shadowed_core') });
+
+    t.equal(res, 'util');
+});
+
 test('shadowed core modules return shadow when appending `/`', function (t) {
     t.plan(2);
 
@@ -19,3 +27,12 @@ test('shadowed core modules return shadow when appending `/`', function (t) {
         t.equal(res, path.join(__dirname, 'shadowed_core/node_modules/util/index.js'));
     });
 });
+
+test('shadowed core modules return shadow when appending `/` [sync]', function (t) {
+    t.plan(1);
+
+    var res = resolve.sync('util/', { basedir: path.join(__dirname, 'shadowed_core') });
+
+    t.equal(res, path.join(__dirname, 'shadowed_core/node_modules/util/index.js'));
+});
+