Bump Dart SDK to `1.18.0-dev.4.0`.

Last dev push for `1.18`.

bin/cache/dart-sdk.version

-1.18.0-dev.3.0
+1.18.0-dev.4.0

packages/flutter_test/pubspec.yaml

@@ -10,7 +10,7 @@ dependencies:
   # We don't actually depend on 'analyzer', but 'test' and 'flutter_tools' do.
   # We pin the version of analyzer we depend on to avoid version skew across our
   # packages.
-  analyzer: 0.27.4-alpha.15
+  analyzer: 0.27.4-alpha.16
 
   flutter:
     path: ../flutter

packages/flutter_tools/pubspec.yaml

@@ -38,7 +38,7 @@ dependencies:
   test: 0.12.13+5
 
   # Pinned in flutter_test as well.
-  analyzer: 0.27.4-alpha.15
+  analyzer: 0.27.4-alpha.16
 
 dev_dependencies:
   mockito: ^0.11.0