Support user generated custom native JS classes.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index ce801a9b8414fb83ec11b204a952f63a9ed69dcb..3463fca87e26eec0093e170de69f1be40160420c 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -289,6 +289,7 @@ class JavaScriptBackend extends Backend {
   ClassElement jsBoolClass;
   ClassElement jsPlainJavaScriptObjectClass;
   ClassElement jsUnknownJavaScriptObjectClass;
+  ClassElement jsJavaScriptFunctionClass;
 
   ClassElement jsIndexableClass;
   ClassElement jsMutableIndexableClass;
@@ -660,7 +661,7 @@ class JavaScriptBackend extends Backend {
   }
 
   FunctionElement resolveExternalFunction(FunctionElement element) {
-    if (isForeign(element)) return element;
+    if (isForeign(element) || element.isJsInterop) return element;
     return patchResolverTask.measure(() {
       return patchResolverTask.resolveExternalFunction(element);
     });
@@ -1039,6 +1040,7 @@ class JavaScriptBackend extends Backend {
         enqueue(enqueuer, getNativeInterceptorMethod, registry);
         enqueueClass(enqueuer, jsInterceptorClass, compiler.globalDependencies);
         enqueueClass(enqueuer, jsPlainJavaScriptObjectClass, registry);
+        enqueueClass(enqueuer, jsJavaScriptFunctionClass, registry);
       } else if (cls == mapLiteralClass) {
         // For map literals, the dependency between the implementation class
         // and [Map] is not visible, so we have to add it manually.
@@ -1119,6 +1121,8 @@ class JavaScriptBackend extends Backend {
       addInterceptors(jsPlainJavaScriptObjectClass, enqueuer, registry);
     } else if (cls == jsUnknownJavaScriptObjectClass) {
       addInterceptors(jsUnknownJavaScriptObjectClass, enqueuer, registry);
+    } else if (cls == jsJavaScriptFunctionClass) {
+      addInterceptors(jsJavaScriptFunctionClass, enqueuer, registry);
     } else if (Elements.isNativeOrExtendsNative(cls)) {
       addInterceptorsForNativeClassMembers(cls, enqueuer);
     } else if (cls == jsIndexingBehaviorInterface) {
@@ -1137,6 +1141,7 @@ class JavaScriptBackend extends Backend {
     Registry registry = compiler.globalDependencies;
     enqueue(enqueuer, getNativeInterceptorMethod, registry);
     enqueueClass(enqueuer, jsPlainJavaScriptObjectClass, registry);
+    enqueueClass(enqueuer, jsJavaScriptFunctionClass, registry);
     needToInitializeIsolateAffinityTag = true;
     needToInitializeDispatchProperty = true;
   }
@@ -1467,6 +1472,7 @@ class JavaScriptBackend extends Backend {
   }
 
   ClassElement defaultSuperclass(ClassElement element) {
+    if (element.isJsInterop) return jsPlainJavaScriptObjectClass;
     // Native classes inherit from Interceptor.
     return element.isNative ? jsInterceptorClass : compiler.objectClass;
   }
@@ -2099,6 +2105,7 @@ class JavaScriptBackend extends Backend {
         jsExtendableArrayClass = findClass('JSExtendableArray');
         jsUnmodifiableArrayClass = findClass('JSUnmodifiableArray');
         jsPlainJavaScriptObjectClass = findClass('PlainJavaScriptObject');
+        jsJavaScriptFunctionClass = findClass('JavaScriptFunction');
         jsUnknownJavaScriptObjectClass = findClass('UnknownJavaScriptObject');
         jsIndexableClass = findClass('JSIndexable');
         jsMutableIndexableClass = findClass('JSMutableIndexable');