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

pkg/compiler/lib/src/js_emitter/old_emitter/class_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/class_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/class_emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/class_emitter.dart
index 6a8381564379138dd1b8e1aed7cc4575122b3c6f..014ee3880087c214f72c03ba156cfb3b4b2f3577 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/class_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/class_emitter.dart
@@ -63,6 +63,9 @@ class ClassEmitter extends CodeEmitterHelper {
         className, classElement, builder, properties);
   }
 
+  /**
+   * [emitClassConstructor] affects the generation of constructors in CSP mode.
+   */
   void emitClassConstructor(ClassElement classElement,
                             ClassBuilder builder,
                             {bool onlyForRti: false}) {
@@ -218,6 +221,11 @@ class ClassEmitter extends CodeEmitterHelper {
     return fieldsAdded;
   }
 
+  /**
+   * [emitClassGettersSetters] does not affect whether or not a class is needed

[floitsch, 2015/01/15 15:14:40]

That comment is out of context.

It should say something like:
If necessary, emits getters and setters for fields.

They can be needed in CSP mode, or in checked mode, when types need to be
verified.

[zarah, 2015/01/16 08:48:50]

You are right. Comment updated.

+   * If getters/setters are emitted, the class has fields and is therefore
+   * non-trivial.
+   */
   void emitClassGettersSetters(ClassElement classElement,
                                ClassBuilder builder,
                                {bool onlyForRti: false}) {