[dart2bytecode] Generate instance field initializers for hot reload

Hot reload needs to run instance field initializers for all newly
added fields except fields initialized with null.

TEST=ci (vm/cc/IsolateReload_RunNewFieldInitializers)

Change-Id: I3ead726aa12b9d6c5aa515c7297deca0ddd9eb9f
Cq-Include-Trybots: luci.dart.try:vm-aot-dyn-linux-debug-x64-try,vm-aot-dyn-linux-product-x64-try,vm-dyn-linux-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/441861
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

pkg/dart2bytecode/lib/bytecode_generator.dart

@@ -3372,8 +3372,14 @@ class BytecodeGenerator extends RecursiveVisitor {
   }
 
   bool _hasNonTrivialInitializer(Field field) {
-    if (field.initializer == null) return false;
-    return !_isTrivialInitializer(field.initializer);
+    final initializer = field.initializer;
+    if (initializer == null) return false;
+    if (options.emitInstanceFieldInitializers && !field.isStatic) {
+      // Hot reload needs initializers for all instance fields
+      // except fields initialized with null.
+      return !_isNullInitializer(initializer);
+    }
+    return !_isTrivialInitializer(initializer);
   }
 
   bool _isTrivialInitializer(Expression? initializer) {
@@ -3392,6 +3398,11 @@ class BytecodeGenerator extends RecursiveVisitor {
     return false;
   }
 
+  bool _isNullInitializer(Expression? initializer) =>
+      initializer is NullLiteral ||
+      (initializer is ConstantExpression &&
+          initializer.constant is NullConstant);
+
   @override
   void visitStaticGet(StaticGet node) {
     final target = node.target;