Fix runtimeType for strings, integers and double: return their interface type.

runtime/lib/object.cc

Index: runtime/lib/object.cc
===================================================================
--- runtime/lib/object.cc	(revision 28266)
+++ runtime/lib/object.cc	(working copy)
@@ -93,6 +93,8 @@
 
 DEFINE_NATIVE_ENTRY(Object_runtimeType, 1) {
   const Instance& instance = Instance::CheckedHandle(arguments->NativeArgAt(0));
+  // Special handling for following types outside this native.
+  ASSERT(!instance.IsString() && !instance.IsInteger() && !instance.IsDouble());
   const Type& type = Type::Handle(instance.GetType());
   // The static type of null is specified to be the bottom type, however, the
   // runtime type of null is the Null type, which we correctly return here.