dart2js cps: Add SetLength instruction.

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

Index: pkg/compiler/lib/src/cps_ir/bounds_checker.dart
diff --git a/pkg/compiler/lib/src/cps_ir/bounds_checker.dart b/pkg/compiler/lib/src/cps_ir/bounds_checker.dart
index 798875f1eed952c1eb98958d0e2a8695e9be7779..22b5770aa040775b7c082b1a0f375d05955607a6 100644
--- a/pkg/compiler/lib/src/cps_ir/bounds_checker.dart
+++ b/pkg/compiler/lib/src/cps_ir/bounds_checker.dart
@@ -704,6 +704,10 @@ class BoundsChecker extends TrampolineRecursiveVisitor implements Pass {
         // after = before - 1
         makeExactSum(lengthAfter, lengthBefore, -1);
         break;
+
+      case BuiltinMethod.SetLength:
+        makeEqual(lengthAfter, getValue(node.arguments[0].definition));
+        break;
     }
   }