[ddc] Enable sound null safety by default

In the expression compiler worker. It appears all uses manually
pass the flag and this is just preparation to remove it in the
future.

Change-Id: Iad0edfbcab28194b4ffcb520db89943c6239753b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388745
Commit-Queue: Nicholas Shahan <[email protected]>
Reviewed-by: Sigmund Cherem <[email protected]>

Author: nshahan

pkg/dev_compiler/lib/src/kernel/expression_compiler_worker.dart

@@ -218,7 +218,7 @@ class ExpressionCompilerWorker {
     Map<ExperimentalFlag, bool> explicitExperimentalFlags = const {},
     Uri? sdkRoot,
     bool trackWidgetCreation = false,
-    bool soundNullSafety = false,
+    bool soundNullSafety = true,
     ModuleFormat moduleFormat = ModuleFormat.amd,
     bool canaryFeatures = false,
     bool enableAsserts = true,