Introduce DartType.unaliased to avoid use of Resolution in the backend.

pkg/compiler/lib/src/use_unused_api.dart

Index: pkg/compiler/lib/src/use_unused_api.dart
diff --git a/pkg/compiler/lib/src/use_unused_api.dart b/pkg/compiler/lib/src/use_unused_api.dart
index 7a9c03c2daa4a5a942f6a75fd394d53bc3435975..3332dedcc8cc16fedf4be9666e92cf6756c04110 100644
--- a/pkg/compiler/lib/src/use_unused_api.dart
+++ b/pkg/compiler/lib/src/use_unused_api.dart
@@ -270,7 +270,7 @@ usedByTests() {
   world.haveAnyCommonSubtypes(null, null);
   typeGraphInferrer.getCallersOf(null);
   dart_types.Types.sorted(null);
-  new dart_types.Types(compiler).copy(compiler);
+  new dart_types.Types(null).copy(null);
   sourceFileProvider.readStringFromUri(null);
 }
 
@@ -305,7 +305,6 @@ useCompiler(dart2jslib.Compiler compiler) {
 }
 
 useTypes() {
-  new dart_types.ResolvedTypedefType(null, null, null).unalias(null);
 }
 
 useCodeEmitterTask(js_emitter.CodeEmitterTask codeEmitterTask) {