Dart SDK roll for 2019-01-09

f4986cc78d Fixed a couple of typos, including LaTeX error
68f77faa96 Specify that extending or implementing Function does nothing.
0f1b9439b5 Use 'no-' prefix to disable an experiment.
de85d1661a Implement DillClass.supertype
2bc015b03a Remove dependency on TypeEnvironment.hierarchy
7ba199794d Save transient state
2bb2f5e834 Simplify ClassHierarchyBuilder.getNode
34afa499a1 Merge interfaces only when necessary
1dc920e0d7 Change names to reflect merge isn't used only for superclasses

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': '50ea6344f5a85493b57df086bd70b686ca2baaf6',
+  'dart_revision': 'f4986cc78ddd884e60e7274c44f31dbcd990b912',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py