[ddc] Handle undefined as null in switch

Add a case for `undefined` whenever a case expression is a null
constant.

Issue: #51527
Change-Id: I6e9946b588e293bcf0b953d568495c8be95f3749
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310775
Commit-Queue: Nicholas Shahan <[email protected]>
Reviewed-by: Stephen Adams <[email protected]>

Author: nshahan

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

@@ -4853,6 +4853,10 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
         expressions.isNotEmpty && !node.isDefault ? expressions.last : null;
     for (var e in expressions) {
       var jsExpr = _visitExpression(e);
+      if (e is ConstantExpression && e.constant is NullConstant) {
+        // Coerce null and undefined by adding an extra case.
+        cases.add(js_ast.Case(js_ast.Prefix('void', js.number(0)), emptyBlock));
+      }
       cases.add(js_ast.Case(jsExpr, e == lastExpr ? body : emptyBlock));
     }
     if (node.isDefault) {