Remove use of eval in dev

This violates Codesandbox' CSP which does not permit unsafe-eval.
Instead, just use window to get the current global.

Fixes #6611

packages/react-router/modules/index.js

@@ -1,22 +1,24 @@
 if (__DEV__) {
-  const global = (1, eval)("this");
-  const key = "__react_router_build__";
-  const buildNames = { cjs: "CommonJS", esm: "ES modules", umd: "UMD" };
+  if (typeof window !== "undefined") {
+    const global = window;
+    const key = "__react_router_build__";
+    const buildNames = { cjs: "CommonJS", esm: "ES modules", umd: "UMD" };
 
-  if (global[key] && global[key] !== process.env.BUILD_FORMAT) {
-    const initialBuildName = buildNames[global[key]];
-    const secondaryBuildName = buildNames[process.env.BUILD_FORMAT];
+    if (global[key] && global[key] !== process.env.BUILD_FORMAT) {
+      const initialBuildName = buildNames[global[key]];
+      const secondaryBuildName = buildNames[process.env.BUILD_FORMAT];
 
-    // TODO: Add link to article that explains in detail how to avoid
-    // loading 2 different builds.
-    throw new Error(
-      `You are loading the ${secondaryBuildName} build of React Router ` +
-        `on a page that is already running the ${initialBuildName} ` +
-        `build, so things won't work right.`
-    );
-  }
+      // TODO: Add link to article that explains in detail how to avoid
+      // loading 2 different builds.
+      throw new Error(
+        `You are loading the ${secondaryBuildName} build of React Router ` +
+          `on a page that is already running the ${initialBuildName} ` +
+          `build, so things won't work right.`
+      );
+    }
 
-  global[key] = process.env.BUILD_FORMAT;
+    global[key] = process.env.BUILD_FORMAT;
+  }
 }
 
 export { default as MemoryRouter } from "./MemoryRouter";