dart2js: don't emit unneeded native info.

sdk/lib/_internal/compiler/js_lib/native_helper.dart

Index: sdk/lib/_internal/compiler/js_lib/native_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/native_helper.dart b/sdk/lib/_internal/compiler/js_lib/native_helper.dart
index bec5bbae9b95df632c4e2979cfb295b7c18b199c..0f02d3c59c1ca6b9c52936e2100043975a38a9dd 100644
--- a/sdk/lib/_internal/compiler/js_lib/native_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/native_helper.dart
@@ -130,6 +130,7 @@ var interceptorsForUncacheableTags;
 
 
 lookupInterceptor(String tag) {
+  if (JS('bool', '!#', interceptorsByTag)) return JS('', 'null');

[floitsch, 2015/02/26 18:12:12]

I'm not sure I like this.
If there are native classes that want to lookup an interceptor they should get
one. With this change they fall back to the "UnknownJavaScriptObject"
interceptor (which works, but doesn't feel right).
I prefer if the 'Program's outputContainsNativeClasses was true, even if we
could remove all native classes. Ideally we set it only if one of the
interceptors that contains the nativeTagInfo is needed.
In general this is academic anyway since normally native classes will need their
interceptors.

[floitsch, 2015/02/26 18:12:12]

I don't understand the `return JS('', 'null')` part.

[zarah, 2015/02/27 09:03:48]

As mentioned in my previous comment, I just think that
outputContainsNativeClasses should be consistent with whether or not we emit
native info.

Another solution was to still emit the interceptorsByTag map but if the native
info is not present we still fall back to the "UnknownJavaScriptObject".

   return propertyGet(interceptorsByTag, tag);
 }
 
@@ -302,6 +303,7 @@ void initNativeDispatchContinue() {
   // tags `TAG`, if `window.TAG` is a (constructor) function, set the dispatch
   // property if the function's prototype to a dispatch record.
   var map = interceptorsByTag;
+  if (JS('bool', '!map')) return;

[floitsch, 2015/02/26 18:12:12]

ditto.

also won't work in minified mode.

[zarah, 2015/02/27 09:03:48]

Acknowledged.

   var tags = JS('JSMutableArray', 'Object.getOwnPropertyNames(#)', map);
 
   if (JS('bool', 'typeof window != "undefined"')) {