[web] Hides that Flutter uses requireJS in debug. (#129032)

Flutter web uses requireJS in `debug` mode to assemble a DDC-compiled app from a bunch of small files ("modules").

This caused that `canvaskit.js` (and all other modules that used a browserify-like loading header) didn't work because they attempted to use the `define` function provided by Flutter's instance of `requireJS` (which kept the defined modules private, rather than as globals on the page, as the users of the JS expected).

A [fix](https://github.com/flutter/engine/pull/27342) was added to `flutter/engine` to trick loaders into *not* using the `requireJS` module loader, but a recent change in the fix's js-interop layer *subtly* changed its JS output on the page (objects went from `undefined` to `null`), causing this:

* https://github.com/flutter/flutter/issues/126131 (and others)

This PR hides a bit of code that is commonly used by module loaders to decide that they may use the `define` function provided by requireJS (so the engine workaround can be removed).

## Next steps

* https://github.com/flutter/engine/pull/42941

## Issues

Partially addresses: https://github.com/flutter/flutter/issues/126131 (and others)

## Tests

* Added a unit test to ensure the `delete` stays
* Manually tested with the Gallery app in `debug` mode with a bunch of user-supplied scripts that currently fail to load.
  * Also tested hot restart as suggested by @nshahan

packages/flutter_tools/lib/src/web/bootstrap.dart

@@ -196,6 +196,12 @@ define("$bootstrapModule", ["$entrypoint", "dart_sdk"], function(app, dart_sdk)
       return dart.getSourceMap(url);
     });
   }
+  // Prevent DDC's requireJS to interfere with modern bundling.
+  if (typeof define === 'function' && define.amd) {
+    // Preserve a copy just in case...
+    define._amd = define.amd;
+    delete define.amd;
+  }
 });
 ''';
 }

packages/flutter_tools/test/general.shard/web/bootstrap_test.dart

@@ -62,6 +62,18 @@ void main() {
     expect(result, matches(regex), reason: 'require.config must have a waitSeconds: 0 config entry');
   });
 
+  test('generateMainModule hides requireJS injected by DDC', () {
+    final String result = generateMainModule(
+      entrypoint: 'foo/bar/main.js',
+      nullAssertions: false,
+      nativeNullAssertions: false,
+    );
+    expect(result, contains('''define._amd = define.amd;'''),
+      reason: 'define.amd must be preserved as _amd so users may restore it if needed.');
+    expect(result, contains('''delete define.amd;'''),
+      reason: "define.amd must be removed so packages don't attempt to use Dart's instance.");
+  });
+
   test('generateMainModule embeds urls correctly', () {
     final String result = generateMainModule(
       entrypoint: 'foo/bar/main.js',