Better messages for optimized index errors.

sdk/lib/core/errors.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.core;
 
 /**
  * Error objects thrown in the case of a program failure.
  *
  * An `Error` object represents a program failure that the programmer
@@ skipping 264 matching lines @@
    *
    * An indexable object is one that has a `length` and a and index-operator
    * `[]` that accepts an index if `0 <= index < length`.
    *
    * If [length] is provided, it is used as the length of the indexable object,
    * otherwise the length is found as `indexable.length`.
    */
   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);
     }
   }
 
   /**
    * Check that a range represents a slice of an indexable object.
    *
    * Throws if the range is not valid for an indexable object with
    * the given [length].
@@ skipping 265 matching lines @@
  * the first time it is read. If evaluating the initializer expression causes
  * another read of the variable, this error is thrown.
  */
 class CyclicInitializationError extends Error {
   final String variableName;
   CyclicInitializationError([this.variableName]);
   String toString() => variableName == null
       ? "Reading static variable during its initialization"
       : "Reading static variable '$variableName' during its initialization";
 }