Fix a performance regressions introduced by r10632.

lib/compiler/implementation/js_backend/backend.dart

Index: lib/compiler/implementation/js_backend/backend.dart
diff --git a/lib/compiler/implementation/js_backend/backend.dart b/lib/compiler/implementation/js_backend/backend.dart
index 699e3f2551923fbd00b2749bca42189afc4a214c..279a983fe8c258172ddc4b13737c1c404866cecd 100644
--- a/lib/compiler/implementation/js_backend/backend.dart
+++ b/lib/compiler/implementation/js_backend/backend.dart
@@ -195,7 +195,7 @@ class JavaScriptBackend extends Backend {
       // of the field is determined by the assignments in the constructor
       // body.
       var constructors = classElement.constructors;
-      if (constructors.head !== null && constructors.tail === null) {
+      if (constructors.head !== null && constructors.tail.isEmpty()) {
         return fieldConstructorSetters[classElement][field];
       } else {
         return HType.UNKNOWN;