dart2js: Better comment for startup-emitter.

pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
index 390ecd684ea42eb5ace5a8127e5759c023f5294b..eef3d309c4ed017d69a29211df38c4259ecfa71a 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart
@@ -31,7 +31,8 @@ part of dart2js.js_emitter.startup_emitter.model_emitter;
 // The code relies on the fact that all Dart code is inside holders. As such
 // we can use "global" names however we want. As long as we don't shadow
 // JavaScript variables (like `Array`) we are free to chose whatever variable
-// names we want. Furthermore, the minifier reduces the names of the variables.
+// names we want. Furthermore, the pretty-printer minifies local variables, thus
+// reducing their size.
 const String mainBoilerplate = '''
 {
 // Declare deferred-initializer global, which is used to keep track of the