dart2js: Only emit constructors of the native class interceptors that are used.

pkg/compiler/lib/src/js_backend/native_emitter.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_backend;
 
 class NativeEmitter {
 
   final Map<Element, ClassBuilder> cachedBuilders;
 
@@ skipping 214 matching lines @@
       generateClassInfo(backend.jsInterceptorClass);
       for (ClassElement classElement in classes) {
         generateClassInfo(classElement);
       }
     }
 
     // Emit the native class interceptors that were actually used.
     for (ClassElement classElement in classes) {
       if (!classElement.isNative) continue;
       if (neededClasses.contains(classElement)) {
+        ClassBuilder builder = builders[classElement];
+
+        // In CSP mode [emitClassConstructor] and [emitClassGettersSetters] emit
+        // code outside the classBuilder. They are called here to avoid emitting
+        // code for unneeded classes.
+        //
+        // Having the calls here (instead of in the classBuilder) is safe:

[ahe, 2015/01/15 08:49:51]

I'm not sure this comment adds value long term. When reading this code, why
would you think that this code should be in a different place?

I think this comment should be sufficient:

/*
In CSP mode [emitClassConstructor] and [emitClassGettersSetters] have a
side-effect on "precompiled" functions in [OldEmitter]. For this reason, it is
important that we don't call these methods before we are certain that a class is
needed.
*/

[zarah, 2015/01/15 15:10:19]

Done.

+        //
+        // [emitClassConstructor] only affects the generation of constructors

[ahe, 2015/01/15 08:49:51]

Shouldn't this be part of emitClassConstructor's documentation comment?

[floitsch, 2015/01/15 14:37:20]

Probably both.

[zarah, 2015/01/15 15:10:19]

Added documentation of the function as well.

+        // in CSP mode.
+        //
+        // [emitClassGettersSetters] does not affect whether or not a class is

[ahe, 2015/01/15 08:49:51]

Ditto.

[floitsch, 2015/01/15 14:37:20]

No. this comment should be here.
It explains why we can delay the call to here, and don't need to run it below.

However it could/should also be in the documentation of the function.

[zarah, 2015/01/15 15:10:19]

Added documentation of the function as well.

+        // needed. If getters/setters are emitted, the class has fields and
+        // is therefore non-trivial.
+        emitterTask.oldEmitter.classEmitter.emitClassConstructor(
+            classElement, builder);
+        emitterTask.oldEmitter.classEmitter.emitClassGettersSetters(
+            classElement, builder);
         // Define interceptor class for [classElement].
         emitterTask.oldEmitter.classEmitter.emitClassBuilderWithReflectionData(
-            backend.namer.getNameOfClass(classElement),
-            classElement, builders[classElement],
+            backend.namer.getNameOfClass(classElement), classElement, builder,
             emitterTask.oldEmitter.getElementDescriptor(classElement));
         emitterTask.oldEmitter.needsClassSupport = true;
       }
     }
   }
 
   /**
    * Computes the native classes that are extended (subclassed) by non-native
    * classes and the set non-mative classes that extend them.  (A List is used
    * instead of a Set for out stability).
@@ skipping 45 matching lines @@
     if (compiler.hasIncrementalSupport) {
       builder = cachedBuilders[classElement];
       if (builder != null) return builder;
       builder = new ClassBuilder(classElement, backend.namer);
       cachedBuilders[classElement] = builder;
     } else {
       builder = new ClassBuilder(classElement, backend.namer);
     }
     builder.superName = superName;
 
-    emitterTask.oldEmitter.classEmitter.emitClassConstructor(
-        classElement, builder);
     bool hasFields = emitterTask.oldEmitter.classEmitter.emitFields(
         classElement, builder, classIsNative: true);
     int propertyCount = builder.properties.length;
-    emitterTask.oldEmitter.classEmitter.emitClassGettersSetters(
-        classElement, builder);
+
     emitterTask.oldEmitter.classEmitter.emitInstanceMembers(
         classElement, builder);
     emitterTask.oldEmitter.typeTestEmitter.emitIsTests(classElement, builder);
 
     if (!hasFields &&
         builder.properties.length == propertyCount &&
         superclass is! MixinApplicationElement) {
       builder.isTrivial = true;
     }
 
@@ skipping 149 matching lines @@
         targetOutput.add(';');
       }
       targetOutput.addBuffer(jsAst.prettyPrint(
           new jsAst.ExpressionStatement(init), compiler));
       targetOutput.add('\n');
     }
 
     targetOutput.add('\n');
   }
 }