Alternative 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..ad5c04c9a50a8b3ec65df0e8af7a346b1acdcb10 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 +120,10 @@ class JsInteropAnalysis {
 
       ClassElement classElement = element;
 
+      // Skip classes that are completely unreachable. This should only happen
+      // when all of jsinterop types are unreachable from main.
+      if (!backend.compiler.world.isImplemented(classElement)) return;
+
       if (!classElement
           .implementsInterface(helpers.jsJavaScriptObjectClass)) {
         backend.reporter.reportErrorMessage(classElement,