Better messages for optimized index errors.

sdk/lib/core/errors.dart

Index: sdk/lib/core/errors.dart
diff --git a/sdk/lib/core/errors.dart b/sdk/lib/core/errors.dart
index 144921a474ac8b98367eb32d50e9c6856c1c63c4..0e99b0b271ce48375f613c2fc9522aad6d0e5019 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -282,7 +282,8 @@ class RangeError extends ArgumentError {
   static void checkValidIndex(int index, var indexable,
                               [String name, int length, String message]) {
     if (length == null) length = indexable.length;
-    if (index < 0 || index >= length) {
+    // Comparing with `0` as receiver produces better dart2js type inference.
+    if (0 > index || index >= length) {
       if (name == null) name = "index";
       throw new RangeError.index(index, indexable, name, message, length);
     }