Roll src/third_party/dart 3d9a356..78ce916 (7 commits)

dart-lang/sdk@78ce916d82 [CFE] Add missing word to spell checking list
dart-lang/sdk@3312fe3d26 [cfe] Fully support expired flags in CFE
dart-lang/sdk@5601eabda6 [CFE] Spell check messages and dart code
dart-lang/sdk@a347973cb2 [vm/ffi] Fix function_gc_test to use correct native for joining a helper thread
dart-lang/sdk@e627d8b852 [nnbd_migration] use postdominators to do hard edge detection in most cases
dart-lang/sdk@b614253cdb Test CFE constants in side-effects test
dart-lang/sdk@6839e0d9f8 Migration: create a simple client of the migration API for early testing.

DEPS

@@ -34,7 +34,7 @@ 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': '3d9a356f6e2ae34da9ec166d0a4b94e6eb95ae49',
+  'dart_revision': '78ce916d8219457c872183ce89c594b045891b48',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py