Fix analyzer warnings in assertions of the TypeExpression CL

TBR=asgerf@google.com

Review URL: https://codereview.chromium.org/1641883002 .

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart

@@ -1865,7 +1865,7 @@ class TypeExpression extends Primitive {
                  List<Primitive> arguments)
       : this.arguments = _referenceList(arguments) {
     assert(kind == TypeExpressionKind.INSTANCE
-           ? dartType == dartType.element.thisType
+           ? dartType == (dartType.element as ClassElement).thisType
            : true);
   }
 

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

@@ -992,7 +992,8 @@ class CodeGenerator extends tree_ir.StatementVisitor
             node.dartType, arguments, registry);
       case tree_ir.TypeExpressionKind.INSTANCE:
         // We expect only flat types for the INSTANCE representation.
-        assert(node.dartType == node.dartType.element.thisType);
+        assert(node.dartType ==
+               (node.dartType.element as ClassElement).thisType);
         registry.registerInstantiatedClass(glue.listClass);
         return new js.ArrayInitializer(arguments);
     }