dart2js: replace calls to codeUnitAt when index has favourable type

sdk/lib/_internal/js_runtime/lib/js_string.dart

Index: sdk/lib/_internal/js_runtime/lib/js_string.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_string.dart b/sdk/lib/_internal/js_runtime/lib/js_string.dart
index 9319b14898a5be09fd9dd7474eb6dcc2ad6bee32..86e2937dc0b4041c7930c1586bbfcc64bdf51299 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_string.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_string.dart
@@ -13,9 +13,14 @@ part of _interceptors;
 class JSString extends Interceptor implements String, JSIndexable {
   const JSString();
 
+  @NoInline()
   int codeUnitAt(int index) {
     if (index is !int) throw diagnoseIndexError(this, index);
     if (index < 0) throw diagnoseIndexError(this, index);
+    return _codeUnitAt(index);
+  }
+
+  int _codeUnitAt(int index) {
     if (index >= length) throw diagnoseIndexError(this, index);
     return JS('JSUInt31', r'#.charCodeAt(#)', this, index);
   }