LibJS: Throw on assignment of an const variable

Was stubbed out as an assert, should be handled with a runtime exception.

Libraries/LibJS/Interpreter.cpp

@@ -145,8 +145,10 @@ void Interpreter::set_variable(const FlyString& name, Value value, bool first_as
 
         auto possible_match = scope.variables.get(name);
         if (possible_match.has_value()) {
-            if (!first_assignment && possible_match.value().declaration_kind == DeclarationKind::Const)
+            if (!first_assignment && possible_match.value().declaration_kind == DeclarationKind::Const) {
-                ASSERT_NOT_REACHED();
+                throw_exception<TypeError>("Assignment to constant variable");
+                return;
+            }
 
             scope.variables.set(move(name), { move(value), possible_match.value().declaration_kind });
             return;

Libraries/LibJS/Tests/variable-declaration.js

@@ -0,0 +1,25 @@
+try {
+
+    const ConstantValue = 1;
+    try {
+        ConstantValue = 2;
+    } catch (e) { 
+        assert(e.name === "TypeError");
+        assert(e.message === "Assignment to constant variable");
+        assert(ConstantValue === 1);
+    }
+
+    // Make sure we can define new constants in inner scopes.
+    //
+    const ConstantValue2 = 1;
+
+    do 
+    {
+        const ConstantValue2 = 2;
+    }
+    while (false)
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}