Initialize `var` bindings in runtime environments with `undefined` (#2860)

Currently in draft, because it breaks some of the annexB [`Block-Level Function Declarations Web Legacy Compatibility Semantics`](https://tc39.es/ecma262/#sec-block-level-function-declarations-web-legacy-compatibility-semantics) tests.

This Pull Request fixes #2795, fixes #2779 and fixes  #2760.

It changes the following:

- Initialize `var` bindings in runtime environments with `undefined`
- Add a missing environment at class construction time.

boa_engine/src/bytecompiler/class.rs

@@ -113,6 +113,15 @@ impl ByteCompiler<'_, '_> {
         self.emit(Opcode::GetFunction, &[index]);
         self.emit_u8(0);
 
+        let class_env: Option<(super::Label, super::Label)> = match class.name() {
+            Some(name) if class.has_binding_identifier() => {
+                self.push_compile_environment(false);
+                self.create_immutable_binding(name, true);
+                Some(self.emit_opcode_with_two_operands(Opcode::PushDeclarativeEnvironment))
+            }
+            _ => None,
+        };
+
         self.emit_opcode(Opcode::Dup);
         if let Some(node) = class.super_ref() {
             self.compile_expr(node, true);
@@ -544,6 +553,13 @@ impl ByteCompiler<'_, '_> {
 
         self.emit_opcode(Opcode::Pop);
 
+        if let Some(class_env) = class_env {
+            let env_info = self.pop_compile_environment();
+            self.patch_jump_with_target(class_env.0, env_info.num_bindings as u32);
+            self.patch_jump_with_target(class_env.1, env_info.index as u32);
+            self.emit_opcode(Opcode::PopEnvironment);
+        }
+
         if !expression {
             self.emit_binding(
                 BindingOpcode::InitVar,

boa_engine/src/environments/compile.rs

@@ -217,4 +217,18 @@ impl CompileTimeEnvironment {
     pub(crate) const fn environment_index(&self) -> usize {
         self.environment_index
     }
+
+    /// Gets the indices of all `var` bindings in this environment.
+    pub(crate) fn var_binding_indices(&self) -> Vec<usize> {
+        self.bindings
+            .iter()
+            .filter_map(|(_, binding)| {
+                if binding.lex {
+                    None
+                } else {
+                    Some(binding.index)
+                }
+            })
+            .collect()
+    }
 }

boa_engine/src/environments/runtime.rs

@@ -481,9 +481,14 @@ impl DeclarativeEnvironmentStack {
 
         let this = this.unwrap_or(JsValue::Null);
 
+        let mut bindings = vec![None; num_bindings];
+        for index in compile_environment.borrow().var_binding_indices() {
+            bindings[index] = Some(JsValue::Undefined);
+        }
+
         self.stack
             .push(Environment::Declarative(Gc::new(DeclarativeEnvironment {
-                bindings: GcRefCell::new(vec![None; num_bindings]),
+                bindings: GcRefCell::new(bindings),
                 compile: compile_environment,
                 poisoned: Cell::new(poisoned),
                 with: Cell::new(with),
@@ -534,9 +539,14 @@ impl DeclarativeEnvironmentStack {
             )
         };
 
+        let mut bindings = vec![None; num_bindings];
+        for index in compile_environment.borrow().var_binding_indices() {
+            bindings[index] = Some(JsValue::Undefined);
+        }
+
         self.stack
             .push(Environment::Declarative(Gc::new(DeclarativeEnvironment {
-                bindings: GcRefCell::new(vec![None; num_bindings]),
+                bindings: GcRefCell::new(bindings),
                 compile: compile_environment,
                 poisoned: Cell::new(poisoned),
                 with: Cell::new(with),