Better messages for optimized index errors.

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

Index: sdk/lib/_internal/compiler/js_lib/js_array.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_array.dart b/sdk/lib/_internal/compiler/js_lib/js_array.dart
index d706b7512780c72e7bc400f4396d384557c79c05..e30e1068d0e913d00fe47107591c880e304e7d3b 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_array.dart
@@ -575,21 +575,21 @@ class JSArray<E> extends Interceptor implements List<E>, JSIndexable {
 
   void set length(int newLength) {
     checkGrowable('set length');
-    if (newLength is !int) throw new ArgumentError(newLength);
+    if (newLength is !int) throw new ArgumentError.value(newLength);

[Lasse Reichstein Nielsen, 2015/06/12 14:24:36]

consider passing "newLength" as second argument.

[sra1, 2015/06/12 22:29:04]

Done.

     if (newLength < 0) throw new RangeError.value(newLength);

[Lasse Reichstein Nielsen, 2015/06/12 14:24:36]

Consider using new RangeError.range(newLength, 0, null, "newLength").
That will document that the value must be non-negative (as is done by
RangeError.checkNotNegative).

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

I have done this for now.

The error feels wordy and indirect, and does not capture the max limit.

RangeError (newLength): Invalid value: Not greater than or equal to 0: -5

I'm considering removing the check and falling back on JS's error. This
less-informative message would permit more optimizations (e.g. generating JS 
a.length = b when all we know about b is that it is an integer).

RangeError: Invalid array length
    at JSArray.dart.JSArray.set$length (out.js:414:25)
    at dart.main (out.js:1170:27)


FWIW the VM's error is a bit misleading.

    [1].length = -3;

gives:
RangeError (index): Invalid value: Not in range 0..1, inclusive: -3
#0      List.[]= (dart:core-patch/growable_array.dart:138)
#1      List.length= (dart:core-patch/growable_array.dart:126)
#2      test (file:///home/sra/play1/bug6i6b.dart:8:7)

     JS('void', r'#.length = #', this, newLength);
   }
 
   E operator [](int index) {
-    if (index is !int) throw new ArgumentError(index);
-    if (index >= length || index < 0) throw new RangeError.value(index);
+    if (index is !int) throw diagnoseIndexError(this, index);
+    if (index >= length || index < 0) throw diagnoseIndexError(this, index);
     return JS('var', '#[#]', this, index);
   }
 
   void operator []=(int index, E value) {
     checkMutable('indexed set');
-    if (index is !int) throw new ArgumentError(index);
-    if (index >= length || index < 0) throw new RangeError.value(index);
+    if (index is !int) throw diagnoseIndexError(this, index);
+    if (index >= length || index < 0) throw diagnoseIndexError(this, index);
     JS('void', r'#[#] = #', this, index, value);
   }