Avoid modifying the set returned by getInterceptorsOn()

pkg/compiler/lib/src/ssa/interceptor_simplifier.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import '../common/codegen.dart' show CodegenWorkItem;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../elements/elements.dart';
 import '../js_backend/backend_helpers.dart' show BackendHelpers;
@@ skipping 214 matching lines @@
         node == dominator.receiver &&
         useCount(dominator, node) == 1) {
       interceptedClasses =
           backend.getInterceptedClassesOn(dominator.selector.name);
 
       // If we found that we need number, we must still go through all
       // uses to check if they require int, or double.
       if (interceptedClasses.contains(helpers.jsNumberClass) &&
           !(interceptedClasses.contains(helpers.jsDoubleClass) ||
               interceptedClasses.contains(helpers.jsIntClass))) {
+        Set<ClassElement> required;
         for (HInstruction user in node.usedBy) {
           if (user is! HInvoke) continue;
           Set<ClassElement> intercepted =
               backend.getInterceptedClassesOn(user.selector.name);
           if (intercepted.contains(helpers.jsIntClass)) {
-            interceptedClasses.add(helpers.jsIntClass);
+            required ??= new Set<ClassElement>();
+            required.add(helpers.jsIntClass);
           }
           if (intercepted.contains(helpers.jsDoubleClass)) {
-            interceptedClasses.add(helpers.jsDoubleClass);
+            required ??= new Set<ClassElement>();
+            required.add(helpers.jsDoubleClass);
           }
         }
+        // Don't modify the result of [backend.getInterceptedClassesOn].
+        if (required != null) {
+          interceptedClasses = interceptedClasses.union(required);
+        }
       }
     } else {
       interceptedClasses = new Set<ClassElement>();
       for (HInstruction user in node.usedBy) {
         if (user is HInvokeDynamic &&
             user.isCallOnInterceptor(compiler) &&
             node == user.receiver &&
             useCount(user, node) == 1) {
           interceptedClasses
               .addAll(backend.getInterceptedClassesOn(user.selector.name));
@@ skipping 151 matching lines @@
       instruction = new HInvokeDynamicMethod(
           selector, mask, inputs, node.instructionType, true);
     }
 
     HBasicBlock block = node.block;
     block.addAfter(node, instruction);
     block.rewrite(node, instruction);
     return true;
   }
 }