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 11c10225418418f3d0057e467765597fa79b10a6..ee108de6e357ae04b1d0e3f5abc964ef269cae38 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_builder.dart
@@ -677,6 +677,11 @@ 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) {