dart2js cps: Add instruction for bounds checks.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
index 84c5200515ff58d2f8c9d1082a5874ac7c05177c..9bb21da4aed0f1289c8c862fc9bedf1dd999ac68 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -666,6 +666,11 @@ class Builder implements cps_ir.Visitor/*<NodeCallback|Node>*/ {
     throw 'Unexpected Refinement node in tree builder';
   }
 
+  @override
+  Expression visitBoundsCheck(cps_ir.BoundsCheck node) {
+    throw 'Unexpected BoundsCheck node in tree builder';
+  }
+
   /********** UNUSED VISIT METHODS *************/
 
   unexpectedNode(cps_ir.Node node) {