test: simplify regression test to single entry file

Consolidate the multi-file test into a single entry.js that sets up
globalThis and validates defines in one file, removing the need for
require() or import ordering concerns.

Co-Authored-By: Claude <noreply@anthropic.com>

src/defines.zig

@@ -155,12 +155,14 @@ pub const DefineData = struct {
         // check for nested identifiers
         var valueSplitter = std.mem.splitScalar(u8, value_str, '.');
         var isIdent = true;
+        var isFirstPart = true;
 
         while (valueSplitter.next()) |part| {
-            if (!js_lexer.isIdentifier(part) or js_lexer.Keywords.has(part)) {
+            if (!js_lexer.isIdentifier(part) or (isFirstPart and js_lexer.Keywords.has(part))) {
                 isIdent = false;
                 break;
             }
+            isFirstPart = false;
         }
 
         if (isIdent) {

test/regression/issue/21200.test.ts

@@ -0,0 +1,32 @@
+import { describe } from "bun:test";
+import { itBundled } from "../../bundler/expectBundled";
+
+describe("bundler", () => {
+  // https://github.com/oven-sh/bun/issues/21200
+  // --define with reserved word property names should produce property access, not string literals
+  itBundled("regression/21200/DefineReservedWordProperty", {
+    files: {
+      "entry.js": /* js */ `
+        globalThis.x = { "import": 1, "export": 2, "class": 3, "function": 4, "var": 5, "default": 6 };
+        globalThis.a = { b: { c: { "import": 7 } } };
+        if (VAR1 !== 1) throw 'fail: VAR1=' + VAR1;
+        if (VAR2 !== 2) throw 'fail: VAR2=' + VAR2;
+        if (VAR3 !== 3) throw 'fail: VAR3=' + VAR3;
+        if (VAR4 !== 4) throw 'fail: VAR4=' + VAR4;
+        if (VAR5 !== 5) throw 'fail: VAR5=' + VAR5;
+        if (VAR6 !== 6) throw 'fail: VAR6=' + VAR6;
+        if (VAR7 !== 7) throw 'fail: VAR7=' + VAR7;
+      `,
+    },
+    define: {
+      VAR1: "x.import",
+      VAR2: "x.export",
+      VAR3: "x.class",
+      VAR4: "x.function",
+      VAR5: "x.var",
+      VAR6: "x.default",
+      VAR7: "a.b.c.import",
+    },
+    run: true,
+  });
+});