Better messages for optimized index errors.

sdk/lib/_internal/compiler/js_lib/js_helper.dart

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 1c6183d4f32f67fa48e559fe73df982f09ffb73f..82d66d0c9fe35359b4f06222ec2536a173fb9331 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -1388,23 +1388,41 @@ class JsCache {
  * for example, if a non-integer index is given to an optimized
  * indexed access.
  */
+@NoInline()
 iae(argument) {
   throw _argumentError(argument);
 }
 
 /**
- * Called by generated code to throw an index-out-of-range exception,
- * for example, if a bounds check fails in an optimized indexed
- * access.  This may also be called when the index is not an integer, in
- * which case it throws an illegal-argument exception instead, like
- * [iae], or when the receiver is null.
+ * Called by generated code to throw an index-out-of-range exception, for
+ * example, if a bounds check fails in an optimized indexed access.  This may
+ * also be called when the index is not an integer, in which case it throws an
+ * illegal-argument exception instead, like [iae], or when the receiver is null.
  */
+@NoInline()
 ioore(receiver, index) {
   if (receiver == null) receiver.length; // Force a NoSuchMethodError.
-  if (index is !int) iae(index);
-  throw new RangeError.value(index);
+  throw diagnoseIndexError(receiver, index);
 }
 
+/**
+ * Diagnoses an indexing error. Returns the ArgumentError or RangeError that
+ * describes the problem.
+ */
+@NoInline()
+Error diagnoseIndexError(indexable, index) {
+  if (index is !int) return new ArgumentError.value(index, 'index');
+  int length = indexable.length;
+  // The following returns the same error that would be thrown by
+  // [RangeError.checkValidIndex] with no optional arguments.

[Lasse Reichstein Nielsen, 2015/06/12 14:24:37]

no optional parameters omitted?
no optional parameters provided?

(parameters can be optional, arguments are there or not, so optional arguments
isn't clear).

You do provide the "name" parameter.

[sra1, 2015/06/12 22:29:05]

Done.

+  if (index < 0 || index >= length) {
+    return new RangeError.index(index, indexable, 'index', null, length);
+  }
+  // The above should always match, but if it does not, use the following.
+  return new RangeError.value(index, 'index');
+}
+
+
 stringLastIndexOfUnchecked(receiver, element, start)
   => JS('int', r'#.lastIndexOf(#, #)', receiver, element, start);