Update constants_2018/constant_types_test to allow type parameters in 'as'.

Change-Id: I37b0413914d6677e2be53fb0ce98c51f81694edb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142568
Reviewed-by: Leaf Petersen <leafp@google.com>

pkg/analyzer/lib/src/dart/constant/potentially_constant.dart

@@ -26,18 +26,6 @@ List<AstNode> getNotPotentiallyConstants(
   return collector.nodes;
 }
 
-/// Return `true` if the [node] is a potentially constant type expression.
-bool isPotentiallyConstantTypeExpression(TypeAnnotation node) {
-  if (node is TypeName) {
-    var element = node.name.staticElement;
-    if (element is TypeParameterElement) {
-      return true;
-    }
-  }
-
-  return isConstantTypeExpression(node);
-}
-
 /// Return `true` if the [node] is a constant type expression.
 bool isConstantTypeExpression(TypeAnnotation node) {
   if (node is TypeName) {
@@ -96,6 +84,18 @@ bool isConstantTypeExpression(TypeAnnotation node) {
   return false;
 }
 
+/// Return `true` if the [node] is a potentially constant type expression.
+bool isPotentiallyConstantTypeExpression(TypeAnnotation node) {
+  if (node is TypeName) {
+    var element = node.name.staticElement;
+    if (element is TypeParameterElement) {
+      return true;
+    }
+  }
+
+  return isConstantTypeExpression(node);
+}
+
 bool _isConstantTypeName(Identifier name) {
   var element = name.staticElement;
   if (element is ClassElement || element is GenericTypeAliasElement) {
@@ -213,8 +213,14 @@ class _Collector {
     }
 
     if (node is IsExpression) {
-      if (!isConstantTypeExpression(node.type)) {
-        nodes.add(node.type);
+      if (isNonNullableByDefault) {
+        if (!isPotentiallyConstantTypeExpression(node.type)) {
+          nodes.add(node.type);
+        }
+      } else {
+        if (!isConstantTypeExpression(node.type)) {
+          nodes.add(node.type);
+        }
       }
       collect(node.expression);
       return;

pkg/analyzer/test/src/dart/constant/potentially_constant_test.dart

@@ -187,7 +187,7 @@ var x = a as int;
 ''', () => _xInitializer(), () => [findNode.simple('a as')]);
   }
 
-  test_asExpression_notConstType() async {
+  test_asExpression_typeParameter() async {
     await _assertNotConst(r'''
 const a = 0;
 class A<T> {
@@ -283,7 +283,7 @@ var x = a is int;
 ''', () => _xInitializer(), () => [findNode.simple('a is')]);
   }
 
-  test_isExpression_notConstType() async {
+  test_isExpression_typeParameter() async {
     await _assertNotConst(r'''
 const a = 0;
 class A<T> {
@@ -908,7 +908,7 @@ var x = 'a';
 class PotentiallyConstantWithNullSafetyTest extends PotentiallyConstantTest
     with WithNullSafetyMixin {
   @override
-  test_asExpression_notConstType() async {
+  test_asExpression_typeParameter() async {
     await _assertConst(r'''
 const a = 0;
 class A<T> {
@@ -916,6 +916,18 @@ class A<T> {
     var x = a as T;
   }
 }
+''', () => _xInitializer());
+  }
+
+  @override
+  test_isExpression_typeParameter() async {
+    await _assertConst(r'''
+const a = 0;
+class A<T> {
+  m() {
+    var x = a is T;
+  }
+}
 ''', () => _xInitializer());
   }
 }

tests/language/constants_2018/constant_types_test.dart

@@ -2,8 +2,9 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// Tests that only constant types are allowed in some positions,
-// not type parameters.
+// Tests that in some positions only constant types are allowed, so not
+// type parameters. But in other positions potentially constant types are
+// allowed, so type parameters.
 
 import "package:expect/expect.dart";
 
@@ -15,11 +16,11 @@ class T<X> {
             [];
   const T.test2(Object o)
       : value = o //
-            as X //# 02: compile-time error
+            as X //# 02: ok
   ;
   const T.test3(Object o)
       : value = o //
-            is X //# 03: compile-time error
+            is X //# 03: ok
   ;
   const T.test4()
       : value = null //