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

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

 // Method to test: function(test)
 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];
 }
 
 main() {
   var wrapper = new Wrapper();
   wrapper[33] = wrapper[1]; // make Wrapper.[]= and [] used more than once.
   print(test(new Wrapper(), int.parse('2')));
 }
 
 class Wrapper {
   final List arr = <bool>[true, false, false, true];
   operator[](int i) => this.arr[i];
   operator[]=(int i, v) {
     if (i > arr.length - 1) arr.length = i + 1;
     return arr[i] = v;
   }
 }