Revert r31212. Some tests fail.

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
===================================================================
--- tests/compiler/dart2js/mock_compiler.dart	(revision 31214)
+++ tests/compiler/dart2js/mock_compiler.dart	(working copy)
@@ -151,12 +151,7 @@
     operator ==(other) => true;
     get hashCode => throw "JSNumber.hashCode not implemented.";
 
-    // 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;
+    _tdivFast(other) => 42;
     _shlPositive(other) => 42;
     _shrBothPositive(other) => 42;
     _shrReceiverPositive(other) => 42;