dart2js cps: Add SetLength instruction.

pkg/compiler/lib/src/cps_ir/bounds_checker.dart

 // Copyright (c) 2015, 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.bounds_checker;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart' show Pass;
 import 'octagon.dart';
 import '../constants/values.dart';
@@ skipping 686 matching lines @@
       case BuiltinMethod.Push:
         // after = before + count
         int count = node.arguments.length;
         makeExactSum(lengthAfter, lengthBefore, count);
         break;
 
       case BuiltinMethod.Pop:
         // after = before - 1
         makeExactSum(lengthAfter, lengthBefore, -1);
         break;
+
+      case BuiltinMethod.SetLength:
+        makeEqual(lengthAfter, getValue(node.arguments[0].definition));
+        break;
     }
   }
 
   @override
   void visitLiteralList(LiteralList node) {
     makeConstant(getLength(node, currentEffectNumber), node.values.length);
   }
 
   // ---------------- INTERIOR EXPRESSIONS --------------------
 
@@ skipping 24 matching lines @@
     }
     return node.body;
   }
 }
 
 /// Lattice representing the known (weak) monotonicity of a loop variable.
 ///
 /// The lattice bottom is represented by `null` and represents the case where
 /// the loop variable never changes value during the loop.
 enum Monotonicity { NotMonotone, Increasing, Decreasing, }