Update Dart revision to a83a69ff3eff845980308779cadbdb80cc03a603. (flutter#3902)

Author: chinmaygarde

DEPS

@@ -30,7 +30,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_fluter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': 'be95b916ddf0af2b204286a438e7169e3bb2c67a',
+  'dart_revision': 'a83a69ff3eff845980308779cadbdb80cc03a603',
 
   'dart_args_tag': '0.13.7',
   'dart_async_tag': 'daf66909019d2aaec1721fc39d94ea648a9fdc1d',

travis/licenses_golden/licenses_dart

@@ -1,4 +1,4 @@
-Signature: b30e56ad696f2fcb5da1a00579e68a5b
+Signature: c931e7e92b0f1f1872a60b8f1a18b0aa
 
 UNUSED LICENSES:
 
@@ -1580,12 +1580,14 @@ FILE: ../../../dart/runtime/bin/sync_socket_patch.dart
 FILE: ../../../dart/runtime/bin/sync_socket_unsupported.cc
 FILE: ../../../dart/runtime/bin/sync_socket_win.cc
 FILE: ../../../dart/runtime/lib/async.cc
+FILE: ../../../dart/runtime/observatory/lib/src/elements/nav/reload.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/singletargetcache_ref.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/singletargetcache_view.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/subtypetestcache_ref.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/subtypetestcache_view.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/unlinkedcall_ref.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/unlinkedcall_view.dart
+FILE: ../../../dart/runtime/observatory/lib/src/models/objects/service.dart
 FILE: ../../../dart/runtime/observatory/lib/src/models/objects/single_target_cache.dart
 FILE: ../../../dart/runtime/observatory/lib/src/models/objects/subtype_test_cache.dart
 FILE: ../../../dart/runtime/observatory/lib/src/models/objects/unlinked_call.dart
@@ -1622,6 +1624,7 @@ FILE: ../../../dart/runtime/vm/stack_trace.h
 FILE: ../../../dart/runtime/vm/zone_text_buffer.cc
 FILE: ../../../dart/runtime/vm/zone_text_buffer.h
 FILE: ../../../dart/sdk/lib/io/sync_socket.dart
+FILE: ../../../dart/sdk/lib/vmservice/named_lookup.dart
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 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