Eliminate old style import from language tests

tests/language/const_var_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.
 // All things regarding constant variables.
 
-#library('const_var');
+library const_var;
 
-#import('const_var_helper.dart', prefix: 'foo');
+import 'const_var_helper.dart' as foo;
 
 const untypedTopLevel = 1;
 const int typedTopLevel = 2;
 const Map<String, String> genericTopLevel = const <String, String>{};
 
 main() {
   const untypedLocal = 3;
   const int typedLocal = 4;
   const Map<String,String> genericLocal = const <String, String>{};
   const [];
   const {};
   const <int>[];
   const <String, int>{};
   const Foo();
   const Foo<int>();
   const foo.Foo();
   const foo.Foo<int>();
 }
 
 class Foo<E> {
   const Foo();
 }