Update parser to not check that all final fields are initialized for external constructors.

runtime/vm/parser.h

Index: runtime/vm/parser.h
diff --git a/runtime/vm/parser.h b/runtime/vm/parser.h
index a3ec91b3bebac3dde05e353be2b49feb3c9f5e97..b2778c10865771a467f195a07646a0c1b0684f81 100644
--- a/runtime/vm/parser.h
+++ b/runtime/vm/parser.h
@@ -325,6 +325,7 @@ class Parser : ValueObject {
                             LocalVariable* receiver,
                             GrowableArray<Field*>* initialized_fields);
   void ParseConstructorRedirection(const Class& cls, LocalVariable* receiver);
+  void CheckNoInitializersInExternal();
   void ParseInitializers(const Class& cls,
                          LocalVariable* receiver,
                          GrowableArray<Field*>* initialized_fields);