dart2js cps: Tolerate refinements in more cases.

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';
 import '../constants/values.dart' as values;
@@ skipping 385 matching lines @@
   // constant time by using union-find data structure.
   for (int i = 0; i < cont.parameters.length; i++) {
     if (invoke.arguments[i].definition != cont.parameters[i]) {
       return false;
     }
   }
 
   return true;
 }
 
+Expression _unfoldDeadRefinements(Expression node) {
+  while (node is LetPrim) {
+    LetPrim let = node;
+    Primitive prim = let.primitive;
+    if (prim.hasAtLeastOneUse || prim is! Refinement) return node;
+    node = node.next;
+  }
+  return node;
+}
+
 bool _isBranchTargetOfUselessIf(Continuation cont) {
   // A useless-if has an empty then and else branch, e.g. `if (cond);`.
   //
   // Detect T or F in
   //
   //     let cont Join() = ...
   //       in let cont T() = Join()
   //                   F() = Join()
   //         in branch condition T F
   //
   if (!cont.hasExactlyOneUse) return false;
   if (cont.firstRef.parent is! Branch) return false;
   Branch branch = cont.firstRef.parent;
+
+  // Are both continuations the same InvokeContinuation on a join?
   Continuation trueCont = branch.trueContinuation.definition;
+  Expression trueBody = _unfoldDeadRefinements(trueCont.body);
+  if (trueBody is! InvokeContinuation) return false;
   Continuation falseCont = branch.falseContinuation.definition;
-  // Are both continuations the same InvokeContinuation on a join?
-  if (trueCont.body is! InvokeContinuation) return false;
-  if (falseCont.body is! InvokeContinuation) return false;
-  InvokeContinuation trueInvoke = trueCont.body;
-  InvokeContinuation falseInvoke = falseCont.body;
+  Expression falseBody = _unfoldDeadRefinements(falseCont.body);
+  if (falseBody is! InvokeContinuation) return false;
+  InvokeContinuation trueInvoke = trueBody;
+  InvokeContinuation falseInvoke = falseBody;
   if (trueInvoke.continuation.definition !=
       falseInvoke.continuation.definition) {
     return false;
   }
   assert(trueInvoke.arguments.length == falseInvoke.arguments.length);
   // Matching zero arguments should be adequate, since isomorphic true and false
   // invocations should result in redundant phis which are removed elsewhere.
   if (trueInvoke.arguments.isNotEmpty) return false;
   return true;
 }
@@ skipping 173 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() {}
 }