Fix warnings.

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 c96cd0eba00473d7d7754bf000249b85ac1b3e01..80bab6e812bf3c1528f468e1c2d129e8231e8d64 100644
--- a/lib/compiler/implementation/js_backend/backend.dart
+++ b/lib/compiler/implementation/js_backend/backend.dart
@@ -39,7 +39,7 @@ class HTypeList {
   final List<HType> types;
 
   HTypeList(int length) : types = new List<HType>(length);
-  const HTypeList.allUnknown() : types = null;
+  const HTypeList.withAllUnknown() : types = null;
 
   factory HTypeList.fromInvocation(HInvoke node, HTypeMap types) {
     bool allUnknown = true;
@@ -58,7 +58,7 @@ class HTypeList {
     return result;
   }
 
-  static const HTypeList ALL_UNKNOWN = const HTypeList.allUnknown();
+  static const HTypeList ALL_UNKNOWN = const HTypeList.withAllUnknown();
 
   bool get allUnknown => types === null;
   int get length => types.length;