Ensure the type inferrer does not infer types for runtime helpers used by the backend.

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

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/compiler.dart	(revision 26173)
+++ sdk/lib/_internal/compiler/implementation/compiler.dart	(working copy)
@@ -222,6 +222,12 @@
   /// Returns true if this element should be retained for reflection even if it
   /// would normally be tree-shaken away.
   bool isNeededForReflection(Element element) => false;
+
+  /// Returns true if global optimizations such as type inferencing
+  /// can apply to this element. One category of elements that do not
+  /// apply is runtime helpers that the backend calls, but the
+  /// optimizations don't see those calls.
+  bool canBeUsedForGlobalOptimizations(Element element);
 }
 
 /**