cpsir: insert guard and force specialization for [] and []=

tests/compiler/dart2js/cps_ir/input/optimize_indexers.dart

Index: tests/compiler/dart2js/cps_ir/input/optimize_indexers.dart
diff --git a/tests/compiler/dart2js/cps_ir/input/optimize_indexers.dart b/tests/compiler/dart2js/cps_ir/input/optimize_indexers.dart
index 084bc34b64571361dd47fbc7914931837895e2f5..8150653aaeee8f78bd7ba70fc25d09f78a34b7c1 100644
--- a/tests/compiler/dart2js/cps_ir/input/optimize_indexers.dart
+++ b/tests/compiler/dart2js/cps_ir/input/optimize_indexers.dart
@@ -4,13 +4,16 @@ import 'package:expect/expect.dart';
 // This example illustrates a case we wish to do better in terms of inlining and
 // code generation.
 //
-// Today this function is compiled without inlining Wrapper.[], JSArray.[] and
-// Wrapper.[]= because:
+// Naively this function would be compiled without inlining Wrapper.[],
+// JSArray.[] and Wrapper.[]= because:
 // JSArray.[] is too big (14 nodes)
 // Wrapper.[] is too big if we force inlining of JSArray (15 nodes)
 // Wrapper.[]= is even bigger (46 nodes)
 //
-// See #25478 for ideas on how to make this better.
+// We now do specialization of [] and []= by adding guards and injecting builtin
+// operators. This made it possible to inline []. We still don't see []= inlined
+// yet, that might require that we improve the inlining counting heuristics a
+// bit.
 @NoInline()
 test(data, x) {
   data[x + 1] = data[x];