Issue 3531. Don't allow using built-in identifier as import prefix

tests/language/pseudo_kw_illegal_test.dart

 // Copyright (c) 2012, 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.
 // Check that we cannot use a pseudo keyword at the class level code.
 
 // Pseudo keywords are not allowed to be used as class names.
 class abstract { }    /// 01: compile-time error
 class assert { }      /// 02: compile-time error

[Brian Wilkerson, 2012/06/12 15:20:02]

Missing "as".

-class call { }        /// 03: compile-time error
 class Dynamic { }     /// 04: compile-time error
 class factory { }     /// 05: compile-time error
 class get { }         /// 06: compile-time error
 class implements { }  /// 07: compile-time error
 class import { }      /// 08: compile-time error

[Brian Wilkerson, 2012/06/12 15:20:02]

Remove "import", "library", and "source".

 class interface { }   /// 09: compile-time error
 class library { }     /// 10: compile-time error
 class negate { }      /// 11: compile-time error
 class operator { }    /// 12: compile-time error
 class set { }         /// 13: compile-time error
 class source { }      /// 14: compile-time error
 class static { }      /// 15: compile-time error
 class typedef { }     /// 16: compile-time error
 
 main() { }