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

tests/compiler/dart2js/cps_ir/expected/optimize_indexers.js

 // Expectation for test: 
 // // 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;
 //   }
 // }
 
 function(data, x) {
-  data.$indexSet(0, J.$add$ns(x, 1), C.JSArray_methods.$index(data.arr, x));
+  var v0 = J.$add$ns(x, 1), v1 = data.arr, v2 = v1.length;
+  if (typeof x !== "number" || Math.floor(x) !== x)
+    return H.iae(x);
+  if (x < 0 || x >= v2)
+    return H.ioore(v1, x);
+  data.$indexSet(0, v0, v1[x]);
 }