Ensure all UnknowConcreteTypes are const so that their equality test is sound.

sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart b/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
index 6c05203f9de40758087d2be4802042771106a075..ab4a32b61029152749fe408bc5723488a4d455e7 100644
--- a/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
@@ -86,7 +86,7 @@ abstract class ConcreteType {
    */
   factory ConcreteType.singleton(int maxConcreteTypeSize, BaseType baseType) {
     if (baseType.isUnknown() || maxConcreteTypeSize < 1) {
-      return new UnknownConcreteType();
+      return const UnknownConcreteType();
     }
     Set<BaseType> singletonSet = new Set<BaseType>();
     singletonSet.add(baseType);