Use entities in the TypeMask interface.

pkg/compiler/lib/src/ssa/type_builder.dart

Index: pkg/compiler/lib/src/ssa/type_builder.dart
diff --git a/pkg/compiler/lib/src/ssa/type_builder.dart b/pkg/compiler/lib/src/ssa/type_builder.dart
index 175523a35203001fb3598186aeec4de080ee89bf..92011677d7bda95c4acd236d868e26a950c40651 100644
--- a/pkg/compiler/lib/src/ssa/type_builder.dart
+++ b/pkg/compiler/lib/src/ssa/type_builder.dart
@@ -29,7 +29,7 @@ class TypeBuilder {
     if (!type.isInterfaceType) return original;
     if (type.isObject) return original;
     // The type element is either a class or the void element.
-    Element element = type.element;
+    ClassElement element = type.element;
     TypeMask mask = new TypeMask.subtype(element, builder.closedWorld);
     return new HTypeKnown.pinned(mask, original);
   }
@@ -230,8 +230,9 @@ class TypeBuilder {
     type = type.unaliased;
     assert(assertTypeInContext(type, original));
     if (type.isInterfaceType && !type.treatAsRaw) {
+      InterfaceType interfaceType = type;
       TypeMask subtype =
-          new TypeMask.subtype(type.element, builder.closedWorld);
+          new TypeMask.subtype(interfaceType.element, builder.closedWorld);
       HInstruction representations =
           buildTypeArgumentRepresentations(type, builder.sourceElement);
       builder.add(representations);