dart2js: don't emit unneeded native info.

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

Index: pkg/compiler/lib/src/js_emitter/native_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/native_emitter.dart b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
index 5749e7903fe34ba65a292e9a74be72a988352bf9..03759c607718a6289f08c1162ef28a6639fbd0f1 100644
--- a/pkg/compiler/lib/src/js_emitter/native_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
@@ -168,10 +168,14 @@ class NativeEmitter {
       }
     }
 
-    // Add properties containing the information needed to construct maps used
-    // by getNativeInterceptor and custom elements.
-    if (compiler.enqueuer.codegen.nativeEnqueuer
-        .hasInstantiatedNativeClasses()) {
+    Set<Class> unneededClasses = classes
+        .where((Class cls) => cls.isNative && !neededClasses.contains(cls))
+        .toSet();
+
+    // If native classes are emitted, add properties containing the information
+    // needed to construct maps used by getNativeInterceptor and custom
+    // elements.
+    if (classes.length != unneededClasses.length) {

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

Explain when/why this can happen.

From what I understand this happens when we only execute methods like 'toString'
or 'hashCode' on the native class (basically the ones provided by 'Object').
(We might invoke others, but they don't count if they are inlined).

Is that correct?

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

It happens in the case you mention, I am not completely sure whether or not it
can happen in other cases as well. 

The point is, however, that in the program builder, this is the condition we use
to set the "outputContainsNativeClasses" field on the model which is used to
decide whether or not to emit the code that handles the native info. Here we
used the method hasInstatiatedNativeClasses() to decide whether we add native
info at all which is inconsistent. So I think the two conditions should be the
same since it does not make sense to emit native info if we are not going to use
it.

       void generateClassInfo(Class cls) {
         // Property has the form:
         //
@@ -218,9 +222,7 @@ class NativeEmitter {
     // (floitsch: was run on every native class.)
     //assert(!classElement.hasBackendMembers);
 
-    return classes
-        .where((Class cls) => cls.isNative && !neededClasses.contains(cls))
-        .toSet();
+    return unneededClasses;
   }
 
   /**