Optimizations on String.concat and String.toString need to make sure the receiver is not null.

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 275 matching lines @@
           if (!compiler.enableTypeAssertions) {
             target = backend.jsArrayAdd;
           }
         }
       } else if (input.isString(compiler)) {
         if (selector.applies(backend.jsStringSplit, compiler)) {
           if (node.inputs[2].isString(compiler)) {
             target = backend.jsStringSplit;
           }
         } else if (selector.applies(backend.jsStringConcat, compiler)) {
-          if (node.inputs[2].isString(compiler)) {
+          // `concat` is turned into a JavaScript '+' so we need to
+          // make sure the receiver is not null.
+          if (node.inputs[2].isString(compiler) && !input.canBeNull()) {
             target = backend.jsStringConcat;
           }
-        } else if (selector.applies(backend.jsStringToString, compiler)) {
+        } 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
         // interceptor. We currently need to keep a
         // HInvokeDynamicMethod and not create a HForeign because
         // HForeign is too opaque for the SsaCheckInserter (that adds a
         // bounds check on removeLast). Once we start inlining, the
@@ skipping 1037 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.
     }
   }
 }