update source_span to 1.4.0

DEPS

 # 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.
 
 vars = {
   # The dart_root is the root of our sdk checkout. This is normally
   # simply sdk, but if using special gclient specs it can be different.
   "dart_root": "sdk",
 
   # We use mirrors of all github repos to guarantee reproducibility and
@@ skipping 91 matching lines @@
   "root_certificates_rev": "@a4c7c6f23a664a37bc1b6f15a819e3f2a292791a",
   "scheduled_test_tag": "@0.12.11",
   "shelf_static_tag": "@0.2.4",
   "shelf_packages_handler_tag": "@1.0.0",
   "shelf_tag": "@0.6.7+2",
   "shelf_web_socket_tag": "@0.2.1",
   "smoke_tag" : "@v0.3.6+2",
   "source_map_stack_trace_tag": "@1.1.4",
   "source_maps-0.9.4_rev": "@38524",
   "source_maps_tag": "@0.10.4",
-  "source_span_tag": "@1.3.1",
+  "source_span_tag": "@1.4.0",
   "stack_trace_tag": "@1.7.2",
   "stream_channel_tag": "@1.6.1",
   "string_scanner_tag": "@1.0.1",
   "sunflower_rev": "@879b704933413414679396b129f5dfa96f7a0b1e",
   "test_reflective_loader_tag": "@0.1.0",
   "test_tag": "@0.12.18+1",
   "typed_data_tag": "@1.1.3",
   "usage_tag": "@v3.0.0+1",
   "utf_tag": "@0.9.0+3",
   "watcher_tag": "@0.9.7+3",
@@ skipping 369 matching lines @@
     # Update the Windows toolchain if necessary.
     'name': 'win_toolchain',
     'pattern': '.',
     'action': ['python', 'sdk/build/vs_toolchain.py', 'update'],
   },
   {
     "pattern": ".",
     "action": ["python", Var("dart_root") + "/tools/generate_buildfiles.py"],
   },
 ]