Fix crash in SSA builder for receiver-less calls. Not sure this is correct.

dart/frog/leg/ssa/builder.dart

Index: dart/frog/leg/ssa/builder.dart
diff --git a/dart/frog/leg/ssa/builder.dart b/dart/frog/leg/ssa/builder.dart
index 40c6d540a7142671fb5e556e4eca931f868a6442..b8bf2f5c3633cc92b0c35e6f46e733557b6316e8 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -1504,8 +1504,10 @@ class SsaBuilder implements Visitor {
       HStatic target = new HStatic(interceptor);
       add(target);
       inputs.add(target);
-      visit(node.receiver);
-      inputs.add(pop());
+      if (node.receiver !== null) {
+        visit(node.receiver);
+        inputs.add(pop());
+      }
       addGenericSendArgumentsToList(node.arguments, inputs);
       push(new HInvokeInterceptor(selector, dartMethodName, false, inputs));
       return;