Roll src/third_party/dart bc99f1c..7b9ab9f (5 commits)

7b9ab9f Add error ranges to more diagnostic tests
3782197 Sort summary_common.dart
3728546 Improvements to null_assertion_ambiguous_test
b0c98d5 Add the new manifest error codes to error codes list
ebdefa3 add new "late" and "required" modifiers

DEPS

@@ -31,7 +31,7 @@ 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': 'bc99f1ca2819c67a5260929b03c3d7ed24943b58',
+  'dart_revision': '7b9ab9f35390d36d32c405a807be31ae3ed0ddb7',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py