Make TypeMask an interface and start hiding implementation details of FlatTypeMask.

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

Index: sdk/lib/_internal/compiler/implementation/ssa/codegen.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/codegen.dart	(revision 22167)
+++ sdk/lib/_internal/compiler/implementation/ssa/codegen.dart	(working copy)
@@ -1699,7 +1699,9 @@
     // lines of the NativeEnqueuerBase.processNativeBehavior method.
     if (type.isUnknown()) return;
     TypeMask mask = type.computeMask(compiler);
-    world.registerInstantiatedClass(mask.base.element, work.resolutionTree);
+    for (ClassElement cls in mask.containedClasses(compiler)) {
+      world.registerInstantiatedClass(cls, work.resolutionTree);
+    }
   }
 
   visitForeign(HForeign node) {