Roll dart sdk to 063c854bc4f55fafc2cf09aad74f0357c4fe69be. (#42263)

Rolls dart sdk to 063c854bc4f55fafc2cf09aad74f0357c4fe69be.

## Pre-launch Checklist

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Objective-C, Java style guides].
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above.
- [X] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
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- [X] All existing and new tests are passing.

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diff --git a/DEPS b/DEPS
index 95404cb..20a88d7 100644
--- a/DEPS
+++ b/DEPS
@@ -48,7 +48,7 @@
   # 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