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

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

Index: pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
diff --git a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
index 0e93623c5fb2ed92aec0b1e231ebeed56e4d7aa1..16a1d15f3665c4198bfe996ea8ea1b05e8b4c694 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -141,6 +141,7 @@ class ShrinkingReducer extends Pass {
     // 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.
@@ -168,6 +169,10 @@ class ShrinkingReducer extends Pass {
     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) {