Revert r16112 from trunk.

dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart

Index: dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
===================================================================
--- dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart	(revision 16161)
+++ dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart	(working copy)
@@ -213,6 +213,6 @@
 }
 
 // Patch for 'identical' function.
-// This is very magical: the compiler knows how to optimize a call to
-// identical, so we can just express identical in terms of itself.
-patch bool identical(Object a, Object b) => identical(a, b);
+patch bool identical(Object a, Object b) {
+  throw new Error('Should not reach the body of identical');
+}