Update range errors to agree on the numbers.

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 0e99b0b271ce48375f613c2fc9522aad6d0e5019..30538071ec3fbde3792917087fe1a2b63df48e3b 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -392,12 +392,13 @@ class IndexError extends ArgumentError implements RangeError {
   String get _errorName => "RangeError";
   String get _errorExplanation {
     assert(_hasValue);
-    String target = Error.safeToString(indexable);
-    var explanation = ": index should be less than $length";
     if (invalidValue < 0) {
-      explanation = ": index must not be negative";
+      return ": index must not be negative";
     }
-    return explanation;
+    if (length == 0) {
+      return ": no indices are valid";
+    }
+    return ": index should be less than $length";
   }
 }