We can only use JavaScript's String.split and String.+ if both inputs are Strings.

sdk/lib/_internal/compiler/implementation/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.
 
 part of ssa;
 
 /**
  * This phase simplifies interceptors in multiple ways:
  *
  * 1) If the interceptor is for an object whose type is known, it
@@ skipping 246 matching lines @@
     HInstruction constant = tryComputeConstantInterceptor(
         node.inputs[1], node.interceptedClasses);
 
     if (constant == null) return false;
 
     Selector selector = node.selector;
     // TODO(ngeoffray): make one shot interceptors know whether
     // they have side effects.
     HInstruction instruction;
     if (selector.isGetter()) {
-      instruction= new HInvokeDynamicGetter(
+      instruction = new HInvokeDynamicGetter(
           selector,
           node.element,
           <HInstruction>[constant, node.inputs[1]],
           false);
     } else if (node.selector.isSetter()) {
       instruction = new HInvokeDynamicSetter(
           selector,
           node.element,
           <HInstruction>[constant, node.inputs[1], node.inputs[2]],
           false);
     } else {
       List<HInstruction> inputs = new List<HInstruction>.from(node.inputs);
       inputs[0] = constant;
       instruction = new HInvokeDynamicMethod(selector, inputs, true);
     }
 
     HBasicBlock block = node.block;
     block.addAfter(node, instruction);
     block.rewrite(node, instruction);
     return true;
   }
 }