Distinguish between malformed and malbounded types (fix issues 12552 and 12554).

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
===================================================================
--- runtime/vm/code_generator.cc	(revision 26566)
+++ runtime/vm/code_generator.cc	(working copy)
@@ -611,6 +611,7 @@
       SubtypeTestCache::CheckedHandle(arguments.ArgAt(5));
   ASSERT(!dst_type.IsDynamicType());  // No need to check assignment.
   ASSERT(!dst_type.IsMalformed());  // Already checked in code generator.
+  ASSERT(!dst_type.IsMalbounded(NULL));  // Already checked in code generator.
   ASSERT(!src_instance.IsNull());  // Already checked in inlined code.
 
   Error& malformed_error = Error::Handle();