Introduce typed selectors for getters and setters also for better tree shaking.

lib/compiler/implementation/ssa/builder.dart

Index: lib/compiler/implementation/ssa/builder.dart
===================================================================
--- lib/compiler/implementation/ssa/builder.dart	(revision 6933)
+++ lib/compiler/implementation/ssa/builder.dart	(working copy)
@@ -2115,8 +2115,7 @@
           self,
           graph.addConstantString(new DartString.literal(name)),
           pop()];
-      push(new HInvokeSuper(const Selector(SelectorKind.INVOCATION, 2),
-                            inputs));
+      push(new HInvokeSuper(Selector.INVOCATION_2, inputs));
       return;
     }
     HInstruction target = new HStatic(element);