Support user generated custom native JS classes.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 3212f9406308c66fd34bf3c33220d4125f53e861..7bef6daf33595504d9e97fdd06f5d35ef3b833a4 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -1060,7 +1060,7 @@ class TransformingVisitor extends LeafVisitor {
     if (target is! FieldElement) return false;
     // TODO(asgerf): Inlining native fields will make some tests pass for the
     // wrong reason, so for testing reasons avoid inlining them.
-    if (target.isNative) return false;
+    if (target.isNative || target.isJsInterop) return false;
     Continuation cont = node.continuation.definition;
     if (node.selector.isGetter) {
       GetField get = new GetField(getDartReceiver(node), target);