VM: Fix checked mode crash (issue 13831).

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
===================================================================
--- runtime/vm/class_finalizer.cc	(revision 29562)
+++ runtime/vm/class_finalizer.cc	(working copy)
@@ -1229,6 +1229,19 @@
           getter.set_result_type(type);
           cls.AddFunction(getter);
           field.set_value(Instance::Handle(Object::sentinel().raw()));
+
+          // Create initializer function.
+          // We don't have the start position of the initializer expression
+          // here, but can compute it from the field identifier position:
+          // The initializer expression starts after the assignment token at +2.
+          const intptr_t initializer_pos = field.token_pos() + 2;
+          const Function& init_function = Function::ZoneHandle(
+              Function::NewStaticInitializer(
+                  String::Handle(field.name()),
+                  type,
+                  cls,
+                  initializer_pos));
+          cls.AddFunction(init_function);
         }
       }
     }