When checking overrides, compare methods structurally

Because they may come from different copies of the same class from different modules

compiler/util/src/org/jetbrains/jet/utils/HashSetUtil.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2010-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.jet.utils;
+
+import com.intellij.util.containers.hash.EqualityPolicy;
+import com.intellij.util.containers.hash.LinkedHashMap;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Map;
+import java.util.Set;
+
+public class HashSetUtil {
+    @NotNull
+    public static <T> Set<T> linkedHashSet(@NotNull Set<T> set,  @NotNull EqualityPolicy<T> policy) {
+        // this implementation of LinkedHashMap doesn't admit nulls as values
+        Map<T, String> map = new LinkedHashMap<T, String>(policy);
+        for (T t : set) {
+            map.put(t, "");
+        }
+        return map.keySet();
+    }
+}