Add -ldl linker flag to Dart shell extensions test shared library link.

runtime/bin/bin.gypi

 # 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.
 
 {
   'variables': {
     'io_cc_file': '<(SHARED_INTERMEDIATE_DIR)/io_gen.cc',
     'json_cc_file': '<(SHARED_INTERMEDIATE_DIR)/json_gen.cc',
     'uri_cc_file': '<(SHARED_INTERMEDIATE_DIR)/uri_gen.cc',
     'utf8_cc_file': '<(SHARED_INTERMEDIATE_DIR)/utf8_gen.cc',
@@ skipping 400 matching lines @@
           ],
           'include_dirs': [
             '.',
           ],
           'sources': [
             'test_extension_linux.cc',
           ],
           'defines': [
             'DART_SHARED_LIB',
           ],
+          'ldflags': [
+            '-ldl',
+          ],
         },
       ],
     }],
   ],
 }