Support user generated custom native JS classes.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 65742316c66ff25e990a162e3b8f69e1f1b68f9f..066fedd54aba5add3c47b60775aa6c15009d513a 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -225,7 +225,8 @@ abstract class ElementX extends Element with ElementCommon {
 
   String _fixedBackendName = null;
   bool _isNative = false;
-  bool get isNative => _isNative;
+  bool isJsInterop = false;
+  bool get isNative => _isNative || isJsInterop;
   bool get hasFixedBackendName => _fixedBackendName != null;
   String get fixedBackendName => _fixedBackendName;
   // Marks this element as a native element.
@@ -2419,7 +2420,7 @@ abstract class BaseClassElementX extends ElementX
     return asInstanceOf(compiler.functionClass) != null || callType != null;
   }
 
-  bool get isNative => nativeTagInfo != null;
+  bool get isNative => nativeTagInfo != null || isJsInterop;
 
   void setNative(String name) {
     // TODO(johnniwinther): Assert that this is only called once. The memory