dart2js cps: Preserve parameter hints during eta and beta reductions.

pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart

 // Copyright (c) 2014, 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.
 
 library dart2js.cps_ir.shrinking_reductions;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 import 'cps_fragment.dart';
 
@@ skipping 123 matching lines @@
     // Replace its invocation with the continuation body.
     InvokeContinuation invoke = cont.firstRef.parent;
     InteriorNode invokeParent = invoke.parent;
 
     cont.body.parent = invokeParent;
     invokeParent.body = cont.body;
 
     // Substitute the invocation argument for the continuation parameter.
     for (int i = 0; i < invoke.arguments.length; i++) {
       cont.parameters[i].replaceUsesWith(invoke.arguments[i].definition);
+      invoke.arguments[i].definition.useElementAsHint(cont.parameters[i].hint);

[Kevin Millikin (Google), 2016/01/12 20:10:05]

This would be clearer if the name useElementAsHint indicated that it was
conditional.  I can't think of an obviously great name.

[asgerf, 2016/01/12 20:21:31]

Now that we have the ??= operator maybe we should just inline it.

     }
 
     // Perform bookkeeping on substituted body and scan for new redexes.
     new _RemovalVisitor(_worklist).visit(invoke);
   }
 
   /// Applies the eta-cont reduction:
   ///   letcont k x = j x in E -> E[j/k].
   /// If k is unused, degenerates to dead-cont.
   void _reduceEtaCont(_ReductionTask task) {
     // Might have been mutated, recheck if reduction is still valid.
     // In the following example, the eta-cont reduction of k1 could have been
     // invalidated by an earlier beta-cont-lin reduction of k0.
     //
     //  letcont k0 x0 = E0 in
     //    letcont k1 x1 = k0 x1 in E1
     if (!_isEtaCont(task.node)) {
       return;
     }
 
     // Remove the continuation.
     Continuation cont = task.node;
     _removeContinuation(cont);
 
     InvokeContinuation invoke = cont.body;
     Continuation wrappedCont = invoke.continuation.definition;
 
+    for (int i = 0; i < cont.parameters.length; ++i) {
+      wrappedCont.parameters[i].useElementAsHint(cont.parameters[i].hint);
+    }
+
     // If the invocation of wrappedCont is escaping, then all invocations of
     // cont will be as well, after the reduction.
     if (invoke.isEscapingTry) {
       Reference current = cont.firstRef;
       while (current != null) {
         InvokeContinuation owner = current.parent;
         owner.isEscapingTry = true;
         current = current.next;
       }
     }
@@ skipping 346 matching lines @@
 
   String toString() => "$kind: $node";
 }
 
 /// A dummy class used solely to mark nodes as deleted once they are removed
 /// from a term.
 class _DeletedNode extends Node {
   accept(_) {}
   setParentPointers() {}
 }