Fix regression on side effects of tdiv.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
===================================================================
--- tests/compiler/dart2js/mock_compiler.dart	(revision 31023)
+++ tests/compiler/dart2js/mock_compiler.dart	(working copy)
@@ -151,7 +151,9 @@
     operator ==(other) => true;
     get hashCode => throw "JSNumber.hashCode not implemented.";
 
-    _tdivFast(other) => 42;
+    // Force side effects on _tdivFast to ensure the optimizers do the
+    // right thing.

[sra1, 2013/12/11 09:45:24]

I'm not sure 'do the right thing' is what we really mean here.
My understanding is that the side effect below models a shortcomming of the
effect analysis on the real library.  This is quite obscure and will become
inaccurate if effect analysis is changed in almost any way.
The comment should say all this, because the code below is very fragile and
unintuitive in its connection to the real code.

Or maybe I misunderstand.

[ngeoffray, 2013/12/16 13:31:41]

No, that's correct. I improved the comment.

+    _tdivFast(other) => new List()..length = 42;
     _shlPositive(other) => 42;
     _shrBothPositive(other) => 42;
     _shrReceiverPositive(other) => 42;