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 a13db64f16ffeb8002c0c9af344c535d073f94c4..70d1a6712913297594ae78fb805f6f359c5423ef 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;
       bool isExtension =
           !isNative && Elements.isNativeOrExtendsNative(classElement);
       // Generate table entries for native classes that are explicitly named and