Update to Dart revision de63e4f

This contains some fixes for Fuchsia and I'd like to keep the version
of Dart used in fuchsia and flutter builds consistent. Should have no
other impactful changes for Flutter.

Author: jamesr

DEPS

@@ -27,7 +27,7 @@ vars = {
 
   # Note: When updating the Dart revision, ensure that all entries that are
   # dependencies of dart are also updated
-  'dart_revision': '1ed4c27c01de6680766593b88a531ef86883a117',
+  'dart_revision': 'de63e4fa94a1cebf86f0541ac14f25fd0bff73c0',
   'dart_boringssl_gen_revision': '1a810313a0290e1caace9da73fa3ab89995ad2c7',
   'dart_boringssl_revision': 'd519bf6be0b447fb80fbc539d4bff4479b5482a2',
   'dart_observatory_packages_revision': '26aad88f1c1915d39bbcbff3cad589e2402fdcf1',