Add a type kind to TypedSelector. A typed selector can either be

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

Index: sdk/lib/_internal/compiler/implementation/ssa/nodes.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/nodes.dart	(revision 18306)
+++ sdk/lib/_internal/compiler/implementation/ssa/nodes.dart	(working copy)
@@ -1152,8 +1152,7 @@
 
     // If the original can't be null, type conversion also can't produce null.
     bool canBeNull = this.guaranteedType.canBeNull();
-    HType convertedType =
-        new HType.fromBoundedType(type, compiler, canBeNull);
+    HType convertedType = new HType.subtype(type, compiler);
 
     // No need to convert if we know the instruction has
     // [convertedType] as a bound.
@@ -2302,7 +2301,7 @@
 }
 
 class HTypeConversion extends HCheck {
-  HType type;
+  final HType type;
   final int kind;
 
   static const int NO_CHECK = 0;
@@ -2313,6 +2312,7 @@
 
   HTypeConversion(this.type, HInstruction input, [this.kind = NO_CHECK])
       : super(<HInstruction>[input]) {
+    assert(type != null);
     sourceElement = input.sourceElement;
   }
   HTypeConversion.checkedModeCheck(HType type, HInstruction input)