Make identical an external function and make it throw in both runtimes, as they both recognize it.

sdk/lib/_internal/compiler/implementation/ssa/builder.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/builder.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(revision 15905)
+++ sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(working copy)
@@ -3370,8 +3370,6 @@
         return;
       }
 
-      // TODO(kasperl): Try to use the general inlining infrastructure for
-      // inlining the identical function.
       if (identical(element, compiler.identicalFunction)) {
         pushWithPosition(new HIdentity(target, inputs[1], inputs[2]), node);
         return;