Changes to make leg build bots green

dart/frog/leg/resolver.dart

Index: dart/frog/leg/resolver.dart
diff --git a/dart/frog/leg/resolver.dart b/dart/frog/leg/resolver.dart
index 81807b501d30a56fc1c61ab8a2df56852f1f4da7..0f8a0e6a722bd65c52d1d431ca2532ce65de69b7 100644
--- a/dart/frog/leg/resolver.dart
+++ b/dart/frog/leg/resolver.dart
@@ -1054,7 +1054,9 @@ class ResolverVisitor extends CommonResolverVisitor<Element> {
       label.setBreakTarget();
       mapping[node.target] = label;
     }
-    assert(mapping[node] === null || mapping[node] === target);
+    if (mapping[node] !== null && mapping[node] !== target) {
+      cancel(node, "internal error");
+    }
     mapping[node] = target;
   }