dart2js cps: Do not inline string interpolation helper.

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

Index: pkg/compiler/lib/src/cps_ir/inline.dart
diff --git a/pkg/compiler/lib/src/cps_ir/inline.dart b/pkg/compiler/lib/src/cps_ir/inline.dart
index d325ed30a0935b9af6e30c35e3f5fd2373a83839..28ea3d5543c4ec50f757fb8766077d249681950e 100644
--- a/pkg/compiler/lib/src/cps_ir/inline.dart
+++ b/pkg/compiler/lib/src/cps_ir/inline.dart
@@ -598,6 +598,7 @@ class InliningVisitor extends TrampolineRecursiveVisitor {
       // These should be handled by operator specialization.
       return true;
     }
+    if (target == backend.helpers.stringInterpolationHelper) return true;
     return false;
   }
 }