Fix checkedInstructionOrNonGenerateAtUseSite.

sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.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;
 
 /**
  * The [LiveRange] class covers a range where an instruction is live.
  */
 class LiveRange {
@@ skipping 207 matching lines @@
 
   void markInputsAsLiveInEnvironment(HInstruction instruction,
                                      LiveEnvironment environment) {
     for (int i = 0, len = instruction.inputs.length; i < len; i++) {
       markAsLiveInEnvironment(instruction.inputs[i], environment);
     }
   }
 
   // Returns the non-HCheck instruction, or the last [HCheck] in the
   // check chain that is not generate at use site.
+  //
+  // For example:
+  //
+  //     t1 = GeneratedAtUseSite instruction
+  //     t2 = check(t1)
+  //     t3 = check(t2)
+  //     t4 = use(t3)
+  //     t5 = use(t3)
+  //     t6 = use(t2)
+  //
+  // The t1 is generate-at-use site, and the live-range must thus be on t2 and
+  // not on the checked instruction t1.
+  // When looking for the checkedInstructionOrNonGenerateAtUseSite of t3 we must
+  // return t2.
   HInstruction checkedInstructionOrNonGenerateAtUseSite(HCheck check) {
     var checked = check.checkedInput;
     while (checked is HCheck) {
       HInstruction next = checked.checkedInput;
       if (generateAtUseSite.contains(next)) break;
       checked = next;
     }
     return checked;
   }
 
@@ skipping 455 matching lines @@
       if (!needsName(input)) {
         names.addAssignment(predecessor, input, phi);
       } else {
         names.addCopy(predecessor, input, phi);
       }
     }
 
     namer.allocateName(phi);
   }
 }