Roll dart sdk to 063c854bc4f55fafc2cf09aad74f0357c4fe69be. (#42263)

Rolls dart sdk to 063c854bc4f55fafc2cf09aad74f0357c4fe69be.

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Author: godofredoc

DEPS

@@ -48,7 +48,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/main/DEPS
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '1618a775995ef6f862b9f05be61a6a8fc23f2e4d',
+  'dart_revision': '063c854bc4f55fafc2cf09aad74f0357c4fe69be',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py