Disable closures in initializers and fail instead of crashing.

frog/leg/ssa/builder.dart

Index: frog/leg/ssa/builder.dart
diff --git a/frog/leg/ssa/builder.dart b/frog/leg/ssa/builder.dart
index 086bc88312d503b6c231df69ca385656e088d155..478124917ba365f509fcb27d4631c4aa2e9e90ce 100644
--- a/frog/leg/ssa/builder.dart
+++ b/frog/leg/ssa/builder.dart
@@ -1239,6 +1239,11 @@ class SsaBuilder implements Visitor {
 
   visitFunctionExpression(FunctionExpression node) {
     ClosureData nestedClosureData = closureDataCache[node];
+    if (nestedClosureData === null) {
+      // TODO(floitsch): we can only assume that the reason for not having a
+      // closure data here is, because the function is inside an initializer.
+      compiler.unimplemented("Closures inside initializers", node: node);
+    }
     assert(nestedClosureData !== null);
     assert(nestedClosureData.closureClassElement !== null);
     ClassElement closureClassElement =