Teach analyzer parser to handle native const factories/const constructors.

pkg/analyzer/lib/src/generated/parser.dart

Index: pkg/analyzer/lib/src/generated/parser.dart
diff --git a/pkg/analyzer/lib/src/generated/parser.dart b/pkg/analyzer/lib/src/generated/parser.dart
index 7e913103634855449dd58c091f34a7a6a2990b6f..b12d26f888fc2c56a0fffacfc3ca62cc08bd5c93 100644
--- a/pkg/analyzer/lib/src/generated/parser.dart
+++ b/pkg/analyzer/lib/src/generated/parser.dart
@@ -6252,7 +6252,8 @@ class Parser {
           parseFunctionBody(true, ParserErrorCode.MISSING_FUNCTION_BODY, false);
       if (constKeyword != null &&
           factoryKeyword != null &&
-          externalKeyword == null) {
+          externalKeyword == null &&
+          body is! NativeFunctionBody) {
         _reportErrorForToken(ParserErrorCode.CONST_FACTORY, factoryKeyword);
       } else if (body is EmptyFunctionBody) {
         if (factoryKeyword != null &&
@@ -6262,7 +6263,7 @@ class Parser {
               ParserErrorCode.FACTORY_WITHOUT_BODY, factoryKeyword);
         }
       } else {
-        if (constKeyword != null) {
+        if (constKeyword != null && body is! NativeFunctionBody) {
           _reportErrorForNode(
               ParserErrorCode.CONST_CONSTRUCTOR_WITH_BODY, body);
         } else if (externalKeyword != null) {