dart2js cps: Fix inlining of stringify.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index c200cc185b34add440c692936863bc1ba9c8ad16..8519565cd1f709b838f504c31de0ef11c6692225 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -2078,14 +2078,18 @@ class TransformingVisitor extends DeepRecursiveVisitor {
         node.replaceUsesWith(argument);
         return new CpsFragment();
       } else if (typeSystem.isDefinitelySelfInterceptor(value.type)) {
-        CpsFragment cps = new CpsFragment(node.sourceInformation);
-        Primitive invoke = cps.invokeMethod(argument,
-            Selectors.toString_,
-            value.type,
-            [cps.makeZero()],
-            CallingConvention.DummyIntercepted);
-        node.replaceUsesWith(invoke);
-        return cps;
+        TypeMask toStringReturn = typeSystem.getInvokeReturnType(
+            Selectors.toString_, value.type);
+        if (typeSystem.isDefinitelyString(toStringReturn)) {
+          CpsFragment cps = new CpsFragment(node.sourceInformation);
+          Primitive invoke = cps.invokeMethod(argument,
+              Selectors.toString_,
+              value.type,
+              [cps.makeZero()],
+              CallingConvention.DummyIntercepted);
+          node.replaceUsesWith(invoke);
+          return cps;
+        }
       }
     } else if (node.target == compiler.identicalFunction) {
       if (node.arguments.length == 2) {