[element model] migrate valid_regexps

Bug: dart-lang/linter#5099
Change-Id: I447de40607f9ef4fd49b80c406b75285b28839f2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387604
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/linter/lib/src/rules/valid_regexps.dart

@@ -35,12 +35,12 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   @override
   void visitInstanceCreationExpression(InstanceCreationExpression node) {
-    var element = node.constructorName.staticElement?.enclosingElement3;
-    if (element?.name == 'RegExp' && element?.library.name == 'dart.core') {
+    var element = node.constructorName.element?.enclosingElement2;
+    if (element == null) return;
+
+    if (element.name == 'RegExp' && (element.library2?.isDartCore ?? false)) {
       var args = node.argumentList.arguments;
-      if (args.isEmpty) {
-        return;
-      }
+      if (args.isEmpty) return;
 
       bool isTrue(Expression e) => e is BooleanLiteral && e.value;