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

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

Index: sdk/lib/_internal/compiler/implementation/ssa/optimize.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(revision 26632)
+++ sdk/lib/_internal/compiler/implementation/ssa/optimize.dart	(working copy)
@@ -293,10 +293,13 @@
             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;
         }
       }