Support user generated custom native JS classes.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js.js_emitter;
 
 class InterceptorStubGenerator {
   final Compiler compiler;
   final Namer namer;
   final JavaScriptBackend backend;
@@ skipping 122 matching lines @@
       statements.add(buildInterceptorCheck(backend.jsBoolClass));
     }
     // TODO(ahe): It might be faster to check for Array before
     // function and bool.
     if (hasArray) {
       statements.add(buildInterceptorCheck(backend.jsArrayClass));
     }
 
     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);
       }''', [
           backend.emitter.constructorAccess(compiler.objectClass),
           backend.emitter
               .staticFunctionAccess(backend.getNativeInterceptorMethod)]));
 
     } else {
       ClassElement jsUnknown = backend.jsUnknownJavaScriptObjectClass;
       if (compiler.codegenWorld
@@ skipping 209 matching lines @@
 
           var map = new jsAst.ObjectInitializer(properties);
           elements.add(map);
         }
       }
     }
 
     return new jsAst.ArrayInitializer(elements);
   }
 }