Set dart defines properly while in debug mode. (#152262)

Turns out just supporting the right value for `kDebugMode` was a lot simpler than I thought. Debug builds used to never go through the build system code path when using `flutter run`, but now that we have wasm this can occur with the run command.

This should address https://github.com/flutter/flutter/issues/148850

dev/integration_tests/web/lib/assertion_test.dart

@@ -0,0 +1,32 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:js_interop';
+
+import 'package:flutter/foundation.dart';
+import 'package:web/web.dart' as web;
+
+Future<void> main() async  {
+  bool executedAssert = false;
+  assert(() {
+    executedAssert = true;
+    return true;
+  }());
+
+  final StringBuffer output = StringBuffer();
+  if (executedAssert == kDebugMode) {
+    output.write('--- TEST SUCCEEDED ---');
+  } else {
+    output.write('--- TEST FAILED ---');
+  }
+
+  await web.window.fetch(
+    '/test-result'.toJS,
+    web.RequestInit(
+      method: 'POST',
+      body: '$output'.toJS,
+    )
+  ).toDart;
+  print(output);
+}