A missing enum case in a switch is a warning, not a compile-time error.

pkg/analyzer/lib/src/generated/error_verifier.dart

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 20462bb6e49b6bb5e7347de979ef9eeb80227d8a..ac73a8ae9336ade93f4240524d79c75b583dd017 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -4280,7 +4280,7 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
       int offset = statement.offset;
       int end = statement.rightParenthesis.end;
       _errorReporter.reportErrorForOffset(
-          CompileTimeErrorCode.MISSING_ENUM_CONSTANT_IN_SWITCH,
+          StaticWarningCode.MISSING_ENUM_CONSTANT_IN_SWITCH,
           offset,
           end - offset,
           [constantNames[i]]);