Support user generated custom native JS classes.

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

Index: pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
index 00394a59b95528fae6efa9a098633ac368922db0..59295d4ec228582b3f8c5786955116fcbb7ddf68 100644
--- a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
@@ -71,6 +71,7 @@ class CustomElementsAnalysis {
     if (!Elements.isNativeOrExtendsNative(classElement)) return;
     if (classElement.isMixinApplication) return;
     if (classElement.isAbstract) return;
+    if (classElement.isJsInterop) return;
     joinFor(enqueuer).instantiatedClasses.add(classElement);
   }
 
@@ -151,6 +152,9 @@ class CustomElementsAnalysisJoin {
     var newActiveClasses = new Set<ClassElement>();
     for (ClassElement classElement in instantiatedClasses) {
       bool isNative = classElement.isNative;
+      // JsInterop classes are opaque interfaces without a concrete
+      // implementation.
+      if (classElement.isJsInterop) continue;

[Siggi Cherem (dart-lang), 2015/10/06 22:38:02]

can this happen given the change you have above in line 74?

[Jacob, 2015/10/13 01:19:23]

nope. Removed and moved comment to the other isJsInterop case

       bool isExtension =
           !isNative && Elements.isNativeOrExtendsNative(classElement);
       // Generate table entries for native classes that are explicitly named and