Support user generated custom native JS classes.

pkg/compiler/lib/src/js_emitter/interceptor_stub_generator.dart

Index: pkg/compiler/lib/src/js_emitter/interceptor_stub_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/interceptor_stub_generator.dart b/pkg/compiler/lib/src/js_emitter/interceptor_stub_generator.dart
index 1da1a032e8a139e1038df5c229835bf24c900560..605f69b3ccdb2fa6ab5a00049d3c887b7be6aae1 100644
--- a/pkg/compiler/lib/src/js_emitter/interceptor_stub_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/interceptor_stub_generator.dart
@@ -140,7 +140,8 @@ class InterceptorStubGenerator {
 
     if (hasNative) {
       statements.add(js.statement(r'''{
-          if (typeof receiver != "object") return receiver;
+          if (typeof receiver != "object"
+              && typeof receiver != "function" ) return receiver;

[sra1, 2015/10/01 20:55:28]

This is a hot path. Make the most of every test.

Can we avoid the very common instanceof test if js functions are never normal
Dart objects?

    if (typeof receiver != "object") {
      if (typeof receiver == "function") return #;
      return receiver;
    }

The additional # placeholder gets

  interceptorFor(jsJavaScriptFunctionClass)

[Jacob, 2015/10/02 20:08:15]

I was concerned about this code as well. Added your optimization.

           if (receiver instanceof #) return receiver;
           return #(receiver);
       }''', [