Stop precomputing ClassElement.isValidMixin.

pkg/analyzer/lib/src/dart/element/element.dart

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index e7b6d5ca6103c70a790da6c592ba89bb32b762c8..509dd4c1b5f544b8a7e16f57469d736a093d0829 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -336,7 +336,22 @@ class ClassElementImpl extends ElementImpl implements ClassElement {
   }
 
   @override
-  bool get isValidMixin => hasModifier(Modifier.MIXIN);
+  bool get isValidMixin {
+    if (!context.analysisOptions.enableSuperMixins) {
+      if (hasReferenceToSuper) {
+        return false;
+      }
+      if (!supertype.isObject) {
+        return false;
+      }
+    }
+    for (ConstructorElement constructor in constructors) {
+      if (!constructor.isSynthetic && !constructor.isFactory) {
+        return false;
+      }
+    }
+    return true;
+  }
 
   @override
   ElementKind get kind => ElementKind.CLASS;
@@ -386,13 +401,6 @@ class ClassElementImpl extends ElementImpl implements ClassElement {
     return null;
   }
 
-  /**
-   * Set whether this class is a valid mixin.
-   */
-  void set validMixin(bool isValidMixin) {
-    setModifier(Modifier.MIXIN, isValidMixin);
-  }
-
   @override
   accept(ElementVisitor visitor) => visitor.visitClassElement(this);