Fix crash https://code.google.com/p/dart/issues/detail?id=11793: only compute a range for a loop if…

sdk/lib/_internal/compiler/implementation/ssa/nodes.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.
 
 part of ssa;
 
 abstract class HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
@@ skipping 602 matching lines @@
     to.usedBy.addAll(from.usedBy);
     from.usedBy.clear();
   }
 
   /**
    * Rewrites all uses of the [from] instruction to using either the
    * [to] instruction, or a [HCheck] instruction that has better type
    * information on [to], and that dominates the user.
    */
   void rewriteWithBetterUser(HInstruction from, HInstruction to) {
+    // BUG(11841): Turn this method into a phase to be run after GVN phases.
     Link<HCheck> better = const Link<HCheck>();
     for (HInstruction user in to.usedBy) {
       if (user == from || user is! HCheck) continue;
       HCheck check = user;
       if (check.checkedInput == to) {
         better = better.prepend(user);
       }
     }
 
     if (better.isEmpty) return rewrite(from, to);
@@ skipping 2079 matching lines @@
   HBasicBlock get start => expression.start;
   HBasicBlock get end {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty);
     return statements.last.end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }