Introduce an ElementTypeMask to recognize simple constraints like "the type of parameter foo is the…

sdk/lib/_internal/compiler/implementation/types/flat_type_mask.dart

Index: sdk/lib/_internal/compiler/implementation/types/flat_type_mask.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/types/flat_type_mask.dart	(revision 24183)
+++ sdk/lib/_internal/compiler/implementation/types/flat_type_mask.dart	(working copy)
@@ -62,8 +62,11 @@
   bool get isEmpty => (flags >> 1) == EMPTY;
   bool get isExact => (flags >> 1) == EXACT;
   bool get isNullable => (flags & 1) != 0;
+
   bool get isUnion => false;
   bool get isContainer => false;
+  bool get isForwarding => false;
+  bool get isElement => false;
 
   // TODO(kasperl): Get rid of these. They should not be a visible
   // part of the implementation because they make it hard to add