Fix for the js-interop crash

pkg/compiler/lib/src/js_backend/js_interop_analysis.dart

Index: pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart b/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
index 65a49c860c848a5b5a6176a510fee0ef5095984a..84d66e8829b91d7826816c7bba57ff77a42d520d 100644
--- a/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
@@ -120,6 +120,12 @@ class JsInteropAnalysis {
 
       ClassElement classElement = element;
 
+      // If we have no indication that the type was instantiated, we don't need
+      // to generate code for it. Note that anonymous types are abstract and
+      // contain only a factory constructor, so they are marked in the world
+      // as being implemented rather than being instantiated.
+      if (!backend.compiler.world.isImplemented(classElement)) return;
+
       if (!classElement
           .implementsInterface(helpers.jsJavaScriptObjectClass)) {
         backend.reporter.reportErrorMessage(classElement,