Implement MethodMirror.metadata.

dart/sdk/lib/_internal/compiler/implementation/enqueue.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 dart2js;
 
 class EnqueueTask extends CompilerTask {
   final ResolutionEnqueuer resolution;
   final CodegenEnqueuer codegen;
 
@@ skipping 213 matching lines @@
         }
         if (universe.hasInvokedSetter(member, compiler)) {
           nativeEnqueuer.registerFieldStore(member);
           // See comment after registerFieldLoad above.
           nativeEnqueuer.registerFieldLoad(member);
           return;
         }
         // Native fields need to go into instanceMembersByName as they
         // are virtual instantiation points and escape points.
       } else {
-        // The codegen inlines instance fields initialization, so it
-        // does not need to add individual fields in the work list.
         if (isResolutionQueue) {
+          // All field initializers must be resolved as they could
+          // have an observable side-effect (and cannot be tree-shaken
+          // away).  However, codegen inlines field initializers, so
+          // it does not need to add individual fields in the work
+          // list.
           addToWorkList(member);
         }
         return;
       }
     } else if (member.kind == ElementKind.FUNCTION) {
       if (member.name == Compiler.NO_SUCH_METHOD) {
         enableNoSuchMethod(member);
       }
       // If there is a property access with the same name as a method we
       // need to emit the method.
@@ skipping 509 matching lines @@
     while(!queue.isEmpty) {
       // TODO(johnniwinther): Find an optimal process order for codegen.
       f(queue.removeLast());
     }
   }
 
   void _logSpecificSummary(log(message)) {
     log('Compiled ${generatedCode.length} methods.');
   }
 }