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

sdk/lib/_internal/compiler/implementation/ssa/optimize.dart

 // Copyright (c) 2012, 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;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 281 matching lines @@
           target = backend.jsArrayRemoveLast;
         } else if (selector.applies(backend.jsArrayAdd, compiler)) {
           // The codegen special cases array calls, but does not
           // inline argument type checks.
           if (!compiler.enableTypeAssertions) {
             target = backend.jsArrayAdd;
           }
         }
       } else if (input.isString(compiler)) {
         if (selector.applies(backend.jsStringSplit, compiler)) {
-          if (node.inputs[2].isString(compiler)) {
+          HInstruction argument = node.inputs[2];
+          if (argument.isString(compiler) && !argument.canBeNull()) {
             target = backend.jsStringSplit;
           }
         } else if (selector.applies(backend.jsStringOperatorAdd, compiler)) {
           // `operator+` is turned into a JavaScript '+' so we need to
-          // make sure the receiver is not null.
-          if (node.inputs[2].isString(compiler) && !input.canBeNull()) {
+          // make sure the receiver and the argument are not null.
+          HInstruction argument = node.inputs[2];
+          if (argument.isString(compiler)
+              && !argument.canBeNull()
+              && !input.canBeNull()) {
             target = backend.jsStringOperatorAdd;
           }
         } else if (selector.applies(backend.jsStringToString, compiler)
                    && !input.canBeNull()) {
           return input;
         }
       }
       if (target != null) {
         // TODO(ngeoffray): There is a strong dependency between codegen
         // and this optimization that the dynamic invoke does not need an
@@ skipping 1228 matching lines @@
       // that knows it is not of a specific Type.
     }
 
     for (HIf ifUser in notIfUsers) {
       changeUsesDominatedBy(ifUser.elseBlock, input, convertedType);
       // TODO(ngeoffray): Also change uses for the then block on a HType
       // that knows it is not of a specific Type.
     }
   }
 }