Use the Scope interface instead of referring to WriteableScope

RELNOTES: N/A

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=155654655

core/src/main/java/com/google/errorprone/bugpatterns/ReferenceEquality.java

@@ -26,7 +26,7 @@
 import com.sun.source.tree.ClassTree;
 import com.sun.source.tree.ExpressionTree;
 import com.sun.source.tree.MethodTree;
-import com.sun.tools.javac.code.Scope.WriteableScope;
+import com.sun.tools.javac.code.Scope;
 import com.sun.tools.javac.code.Symbol;
 import com.sun.tools.javac.code.Symbol.MethodSymbol;
 import com.sun.tools.javac.code.Type;
@@ -106,7 +106,7 @@ public static boolean implementsEquals(Type type, VisitorState state) {
       if (ASTHelpers.isSameType(sup, state.getSymtab().objectType, state)) {
         return false;
       }
-      WriteableScope scope = sup.tsym.members();
+      Scope scope = sup.tsym.members();
       if (scope == null) {
         continue;
       }