dart2js: strengthen loop condition in loop body

pkg/compiler/lib/src/ssa/optimize.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.
 
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart' show Selectors;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 2375 matching lines @@
     // and we don't want "if (x==null) return x;" to convert between JavaScript
     // 'null' and 'undefined'.
   }
 
   collectTargets(HInstruction instruction, List<HBasicBlock> trueTargets,
       List<HBasicBlock> falseTargets) {
     for (HInstruction user in instruction.usedBy) {
       if (user is HIf) {
         trueTargets?.add(user.thenBlock);
         falseTargets?.add(user.elseBlock);
+      } else if (user is HLoopBranch) {
+        trueTargets?.add(user.block.successors.first);
+        // Don't insert refinements on else-branch - may be a critical edge
+        // block which we currently need to keep empty (except for phis).
       } else if (user is HNot) {
         collectTargets(user, falseTargets, trueTargets);
       } else if (user is HPhi) {
         List<HInstruction> inputs = user.inputs;
         if (inputs.length == 2) {
           assert(inputs.contains(instruction));
           HInstruction other = inputs[(inputs[0] == instruction) ? 1 : 0];
           if (other.isConstantTrue()) {
             // The condition flows to a HPhi(true, user), which means that a
             // downstream HIf has true-branch control flow that does not depend
@@ skipping 576 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }