LibJS: Fix assignment of const variable on declaration

We were previously assuming that we were reassigning a variable even
when we were not, oops, my bad. :/

Libraries/LibJS/AST.cpp

@@ -535,7 +535,7 @@ Value VariableDeclaration::execute(Interpreter& interpreter) const
     interpreter.declare_variable(name().string(), m_declaration_type);
     if (m_initializer) {
         auto initalizer_result = m_initializer->execute(interpreter);
-        interpreter.set_variable(name().string(), initalizer_result);
+        interpreter.set_variable(name().string(), initalizer_result, true);
     }
 
     return js_undefined();

Libraries/LibJS/Interpreter.cpp

@@ -108,14 +108,14 @@ void Interpreter::declare_variable(String name, DeclarationType declaration_type
     }
 }
 
-void Interpreter::set_variable(String name, Value value)
+void Interpreter::set_variable(String name, Value value, bool first_assignment)
 {
     for (ssize_t i = m_scope_stack.size() - 1; i >= 0; --i) {
         auto& scope = m_scope_stack.at(i);
 
         auto possible_match = scope.variables.get(name);
         if (possible_match.has_value()) {
-            if (possible_match.value().declaration_type == DeclarationType::Const)
+            if (!first_assignment && possible_match.value().declaration_type == DeclarationType::Const)
                 ASSERT_NOT_REACHED();
 
             scope.variables.set(move(name), { move(value), possible_match.value().declaration_type });

Libraries/LibJS/Interpreter.h

@@ -71,7 +71,7 @@ public:
     void do_return();
 
     Value get_variable(const String& name);
-    void set_variable(String name, Value);
+    void set_variable(String name, Value, bool first_assignment = false);
     void declare_variable(String name, DeclarationType);
 
     void gather_roots(Badge<Heap>, HashTable<Cell*>&);