Fix regression on side effects of tdiv.

tests/compiler/dart2js/mock_compiler.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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 133 matching lines @@
     operator &(other) => 42;
     operator ^(other) => 42;
 
     operator >(other) => true;
     operator >=(other) => true;
     operator <(other) => true;
     operator <=(other) => true;
     operator ==(other) => true;
     get hashCode => throw "JSNumber.hashCode not implemented.";
 
-    _tdivFast(other) => 42;
+    // We force side effects on _tdivFast to mimic the shortcomings of
+    // the effect analysis: because the `_tdivFast` implementation of
+    // the core library has calls that may not already be analyzed,
+    // the analysis will conclude that `_tdivFast` may have side
+    // effects.
+    _tdivFast(other) => new List()..length = 42;
     _shlPositive(other) => 42;
     _shrBothPositive(other) => 42;
     _shrReceiverPositive(other) => 42;
     _shrOtherPositive(other) => 42;
 
     abs() => (this is JSInt) ? 42 : 42.2;
     remainder(other) => (this is JSInt) ? 42 : 42.2;
     truncate() => 42;
   }
   class JSInt extends JSNumber implements int {
@@ skipping 393 matching lines @@
     } else {
       sourceFile = compiler.sourceFiles[uri.toString()];
     }
     if (sourceFile != null && begin != null && end != null) {
       print(sourceFile.getLocationMessage(message, begin, end, true, (x) => x));
     } else {
       print(message);
     }
   };
 }