Prefer require.cache over native module cache

This patch changes node's module loading behavior so that the
require.cache is always the first place to consider when loading a
module.

The motivation for this change is to help people who are writing
focused tests for their node.js applications, and need a mechanism
to inject test doubles to replace native node.js modules.

src/node.js

@@ -238,6 +238,9 @@
       var id = resolved[0];
       var filename = resolved[1];
 
+      var cachedModule = moduleCache[filename];
+      if (cachedModule) return cachedModule.exports;
+
       // With natives id === request
       // We deal with these first
       if (natives[id]) {
@@ -253,9 +256,6 @@
         return requireNative(id);
       }
 
-      var cachedModule = moduleCache[filename];
-      if (cachedModule) return cachedModule.exports;
-
       var module = new Module(id, parent);
       moduleCache[filename] = module;
       module.load(filename);

test/simple/test-require-cache.js

@@ -0,0 +1,22 @@
+var common = require('../common');
+var assert = require('assert');
+
+(function testInjectFakeModule() {
+  var relativePath = '../fixtures/semicolon';
+  var absolutePath = require.resolve(relativePath);
+  var fakeModule = {};
+
+  require.cache[absolutePath] = {exports: fakeModule};
+
+  assert.strictEqual(require(relativePath), fakeModule);
+})();
+
+
+(function testInjectFakeNativeModule() {
+  var relativePath = 'fs';
+  var fakeModule = {};
+
+  require.cache[relativePath] = {exports: fakeModule};
+
+  assert.strictEqual(require(relativePath), fakeModule);
+})();