dart2js: don't emit unneeded native info.

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

 // Copyright (c) 2012, 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 _js_helper;
 
 
 // TODO(ngeoffray): stop using this method once our optimizers can
 // change str1.contains(str2) into str1.indexOf(str2) != -1.
 bool contains(String userAgent, String name) {
@@ skipping 112 matching lines @@
 var dispatchRecordsForInstanceTags;
 
 /**
  * Cache of interceptors indexed by uncacheable tags, e.g. "~SomeThing".
  * This is a JavaScript object used as a map.
  */
 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);
 }
 
 
 // Dispatch tag marks are optional prefixes for a dispatch tag that direct how
 // the interceptor for the tag may be cached.
 
 /// No caching permitted.
 const UNCACHED_MARK = '~';
 
@@ skipping 152 matching lines @@
 
   dispatchRecordsForInstanceTags = JS('', 'Object.create(null)');
   interceptorsForUncacheableTags = JS('', 'Object.create(null)');
 
   initHooks();
 
   // Try to pro-actively patch prototypes of DOM objects.  For each of our known
   // 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"')) {
     var context = JS('=Object', 'window');
     var fun = JS('=Object', 'function () {}');
     for (int i = 0; i < tags.length; i++) {
       var tag = tags[i];
       var proto = prototypeForTagFunction(tag);
       if (proto != null) {
         var interceptorClass = JS('', '#[#]', map, tag);
@@ skipping 328 matching lines @@
 const _safariHooksTransformer = const JS_CONST(r'''
 function(hooks) { return hooks; }
 ''');
 
 
 const _dartExperimentalFixupGetTagHooksTransformer = const JS_CONST(r'''
 function(hooks) {
   if (typeof dartExperimentalFixupGetTag != "function") return hooks;
   hooks.getTag = dartExperimentalFixupGetTag(hooks.getTag);
 }''');