Refactor qualified send sets.

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index 9fadfe2823bcc8cbf3b821cb4a5d87071f6e2552..4a2f3e0de7ed64455e5387e51c9ff41b48e6a96d 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -3628,11 +3628,13 @@ class SsaBuilder extends ast.Visitor
                                  Element element,
                                  HInstruction value,
                                  {ast.Node location}) {
-    assert(send == null || !Elements.isInstanceSend(send, elements));
     if (location == null) {
       assert(send != null);
       location = send;
     }
+    assert(invariant(location,
+        send == null || !Elements.isInstanceSend(send, elements),
+        message: "Unexpected non instance setter: $element."));
     if (Elements.isStaticOrTopLevelField(element)) {
       if (element.isSetter) {
         pushInvokeStatic(location, element, <HInstruction>[value]);