Roll Dart to e673ff0b253d8cadd3249aa0d8efe0516606d704 (#3994)

- Roll Dart to e673ff0b253d8cadd3249aa0d8efe0516606d704
- Update licenses_dart

DEPS

@@ -30,7 +30,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': 'b8bce019b3e4a344d22ac3c0f805b619565a1f8e',
+  'dart_revision': 'e673ff0b253d8cadd3249aa0d8efe0516606d704',
 
   'dart_args_tag': '0.13.7',
   'dart_async_tag': 'daf66909019d2aaec1721fc39d94ea648a9fdc1d',

travis/licenses_golden/licenses_dart

-Signature: 0a861f38b7c82276a0a7e925bdefb8c9
+Signature: f254df4d65693a4e1871b2a3359c82be
 
 UNUSED LICENSES:
 
@@ -1558,9 +1558,11 @@ 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/memory/allocations.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/memory/dashboard.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/memory/graph.dart
 FILE: ../../../dart/runtime/observatory/lib/src/elements/memory/profile.dart
+FILE: ../../../dart/runtime/observatory/lib/src/elements/memory/snapshot.dart
 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