Optimize runtime code on hot methods

pkg/dev_compiler/tool/input_sdk/private/js_array.dart

Index: pkg/dev_compiler/tool/input_sdk/private/js_array.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_array.dart b/pkg/dev_compiler/tool/input_sdk/private/js_array.dart
index dc9564eb0e507b5343d63b1585d9d1f4a308ea70..260f58d79819ec38525a6180cb72b3fb9d81586c 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_array.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_array.dart
@@ -558,15 +558,22 @@ class JSArray<E> implements List<E>, JSIndexable<E> {
   }
 
   E operator [](int index) {
-    if (index is! int) throw diagnoseIndexError(this, index);
-    if (index >= length || index < 0) throw diagnoseIndexError(this, index);
+    // Suppress redundant null checks via JS.
+    if (index == null ||

[ochafik, 2017/06/08 17:23:08]

I wonder how hard it would be to implement naive flow-guided nullability
inference?

(although here it would only optimize the index nullchecks away)

Are there easy-to-use benchmarks for DDC?

+        JS('int', '#', index) >= JS('int', '#.length', this) ||
+        JS('int', '#', index) < 0) {
+      throw diagnoseIndexError(this, index);
+    }
     return JS('var', '#[#]', this, index);
   }
 
   void operator []=(int index, E value) {
     checkMutable('indexed set');
-    if (index is! int) throw diagnoseIndexError(this, index);
-    if (index >= length || index < 0) throw diagnoseIndexError(this, index);
+    if (index == null ||
+        JS('int', '#', index) >= JS('int', '#.length', this) ||
+        JS('int', '#', index) < 0) {
+      throw diagnoseIndexError(this, index);
+    }
     JS('void', r'#[#] = #', this, index, value);
   }