Make List an abstract class.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 88a82794784968c9777d755951e7b64179c6da75..327389133ef1b423f0f7f4d57ee01644968ad6b1 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -8778,9 +8778,7 @@ AstNode* Parser::ParseListLiteral(intptr_t type_pos,
     return new LiteralNode(literal_pos, const_list);
   } else {
     // Factory call at runtime.
-    // TODO(regis): Once _ListImpl is removed, use Symbols::List() instead of
-    // Symbols::ListImplementation() on the following line.
-    String& factory_class_name = String::Handle(Symbols::ListImplementation());
+    String& factory_class_name = String::Handle(Symbols::List());
     const Class& factory_class =
         Class::Handle(LookupCoreClass(factory_class_name));
     ASSERT(!factory_class.IsNull());