dart2js CPS: Remove an invalid assertion in shrinking 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 acb5b91637f0d846bebfd8cf728a91f9dc3a33fb..298a0827b70efffc4657f8ff346733bd0817003f 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -545,11 +545,15 @@ bool _isUselessIf(Branch branch) {
       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;
+  //
+  // Note that the argument lists are not necessarily the same length here,
+  // because we could be looking for new redexes in the middle of performing a
+  // dead parameter reduction, where some but not all of the invocations have
+  // been rewritten.  In that case, we will find the redex (once) after both
+  // of these invocations have been rewritten.
+  return trueInvoke.arguments.isEmpty && falseInvoke.arguments.isEmpty;
 }
 
 bool _isDeadParameter(Parameter parameter) {