Always parse 'x.y' as a prefixed identifier.

pkg/analyzer/test/generated/parser_fasta_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart' as analyzer;
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/generated/parser.dart' as analyzer;
 import 'package:analyzer/src/generated/utilities_dart.dart';
@@ skipping 418 matching lines @@
   @override
   @failingTest
   void
       test_parseAssignableExpression_identifier_args_dot_typeParameterComments() {
     super
         .test_parseAssignableExpression_identifier_args_dot_typeParameterComments();
   }
 
   @override
   @failingTest
+  void test_parseAssignableExpression_identifier_dot() {
+    super.test_parseAssignableExpression_identifier_dot();
+  }
+
+  @override
+  @failingTest
   void test_parseAssignableExpression_identifier_question_dot() {
     super.test_parseAssignableExpression_identifier_question_dot();
   }
 
   @override
   @failingTest
   void test_parseAssignableSelector_dot() {
     super.test_parseAssignableSelector_dot();
   }
 
@@ skipping 1425 matching lines @@
   void test_parseTypeAlias_genericFunction_typeParameters_voidReturnType() {
     super.test_parseTypeAlias_genericFunction_typeParameters_voidReturnType();
   }
 
   @override
   @failingTest
   void test_parseTypeAlias_genericFunction_voidReturnType() {
     super.test_parseTypeAlias_genericFunction_voidReturnType();
   }
 }