Set default Dart SDK to 2.0.0-dev.56.0.

Also remove unnecessary casts with this SDK upgrade.

tap:https://test.corp.google.com/ui#id=OCL:197670958:BASE:198431238:1527619491777:9000b105

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198445295
diff --git a/test/find_default_locale_browser_test.dart b/test/find_default_locale_browser_test.dart
index fbbdcd1..818194e 100644
--- a/test/find_default_locale_browser_test.dart
+++ b/test/find_default_locale_browser_test.dart
@@ -15,7 +15,7 @@
     // should find a way to force the system locale before the test is run
     // and then verify that it's actually the correct value.
     Intl.systemLocale = 'xx_YY';
-    ThenArgument callback = expectAsync1(verifyLocale) as ThenArgument;
+    ThenArgument callback = expectAsync1(verifyLocale);
     findSystemLocale().then(callback);
   });
 }
diff --git a/test/find_default_locale_standalone_test.dart b/test/find_default_locale_standalone_test.dart
index 033ad46..21bf17b 100644
--- a/test/find_default_locale_standalone_test.dart
+++ b/test/find_default_locale_standalone_test.dart
@@ -14,7 +14,7 @@
     // and then verify that it's actually the correct value.
     // We have no way of getting this reliably for Windows, so it will fail.
     Intl.systemLocale = "xx_YY";
-    var callback = expectAsync1(verifyLocale) as ThenArgument;
+    var callback = expectAsync1(verifyLocale);
     findSystemLocale().then(callback);
   });
 }