Do not show mixins in the extends clause

Change-Id: I1e1d757958cb12fc25e2805524cf3fb325634973
Reviewed-on: https://dart-review.googlesource.com/73690
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer_plugin/lib/src/utilities/completion/optype.dart

@@ -540,6 +540,7 @@ class _OpTypeAstVisitor extends GeneralizingAstVisitor {
   void visitExtendsClause(ExtendsClause node) {
     if (identical(entity, node.superclass)) {
       optype.includeTypeNameSuggestions = true;
+      optype.typeNameSuggestionsFilter = _nonMixinClasses;
     }
   }
 
@@ -1036,4 +1037,18 @@ class _OpTypeAstVisitor extends GeneralizingAstVisitor {
     }
     return false;
   }
+
+  /**
+   * A filter used to disable everything except classes (such as functions and
+   * mixins).
+   */
+  int _nonMixinClasses(DartType type, int relevance) {
+    if (type is InterfaceType) {
+      if (type.element.isMixin) {
+        return null;
+      }
+      return relevance;
+    }
+    return null;
+  }
 }