Use entities in BackendClasses

pkg/compiler/lib/src/types/map_type_mask.dart

Index: pkg/compiler/lib/src/types/map_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/map_type_mask.dart b/pkg/compiler/lib/src/types/map_type_mask.dart
index 148fdbf58d7d051156eff4fa44bfc2b53b591cd1..a5199962cd5871fe57800d80fa0342de8c423272 100644
--- a/pkg/compiler/lib/src/types/map_type_mask.dart
+++ b/pkg/compiler/lib/src/types/map_type_mask.dart
@@ -81,7 +81,7 @@ class MapTypeMask extends ForwardingTypeMask {
       // doesn't need the compiler.
       assert(other.keyType ==
           new TypeMask.nonNullExact(
-              closedWorld.backendClasses.stringImplementation, closedWorld));
+              closedWorld.backendClasses.stringClass, closedWorld));
       TypeMask newKeyType = keyType.union(other.keyType, closedWorld);
       TypeMask newValueType =
           other.typeMap.values.fold(keyType, (p, n) => p.union(n, closedWorld));