Don't call stringify if we can prove that the type is a string.

pkg/compiler/lib/src/ssa/kernel_string_builder.dart

Index: pkg/compiler/lib/src/ssa/kernel_string_builder.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_string_builder.dart b/pkg/compiler/lib/src/ssa/kernel_string_builder.dart
index 21da2e4b9afd51bad449697733336bc521b97629..38a3f639dccea316d7cf68a894be393869ce2718 100644
--- a/pkg/compiler/lib/src/ssa/kernel_string_builder.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_string_builder.dart
@@ -43,9 +43,10 @@ class KernelStringBuilder extends ir.Visitor {
       return;
     }
 
-    // TODO(het): If toString method is guaranteed to return a string, then call
-    // it directly instead of stringify. Or, better yet, do this later in the
-    // optimization phase.
+    // TODO(efortuna): If we decide to do inlining before finishing constructing
+    // the control flow graph, we'd want to do the optimization of
+    // calling toString here if the type is provably a string rather than in the
+    // optimization phase (which is where we currently do it).
 
     append(stringify(expression));
   }