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

runtime/vm/dart_api_impl_test.cc

Index: runtime/vm/dart_api_impl_test.cc
diff --git a/runtime/vm/dart_api_impl_test.cc b/runtime/vm/dart_api_impl_test.cc
index 0f613b503412a40a807c9691f00427a0b4b73ae2..e7fb2bd50d583d17a1d67d6983d309a534dd7f9f 100644
--- a/runtime/vm/dart_api_impl_test.cc
+++ b/runtime/vm/dart_api_impl_test.cc
@@ -5479,7 +5479,9 @@ TEST_CASE(ParsePatchLibrary) {
   const char* kLibraryChars =
   "#library('patched_library');\n"
   "class A {\n"
+  "  final int _final;\n"
   "  var _f;\n"
+  "  external A();"
   "  external method(var value);\n"
   "  get field() => _field;\n"
   "}\n"
@@ -5491,6 +5493,7 @@ TEST_CASE(ParsePatchLibrary) {
   const char* kPatchChars =
   "patch class A {\n"
   "  var _g;\n"
+  "  A() : _final = 42;"
   "  get _field() => _g;\n"
   "  patch method(var value) {\n"
   "    _g = value * 5;\n"