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

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 25619fb6330fc37458b7a5bf945509886f773f53..cc0e931027018b570332ccb5c693e3c21b2cb82b 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1309,7 +1309,7 @@ class JavaScriptBackend extends Backend {
                                  Enqueuer world,
                                  Registry registry) {
     assert(!registry.isForResolution);
-    type = type.unalias(resolution);
+    type = type.unaliased;
     enqueueClass(world, compiler.boolClass, registry);
     bool inCheckedMode = compiler.enableTypeAssertions;
     // [registerIsCheck] is also called for checked mode checks, so we
@@ -3234,7 +3234,8 @@ class JavaScriptResolutionCallbacks extends ResolutionCallbacks {
   // TODO(johnniwinther): Maybe split this into [onAssertType] and [onTestType].
   void onIsCheck(DartType type, Registry registry) {
     assert(registry.isForResolution);
-    type = type.unalias(backend.resolution);
+    type.computeUnaliased(backend.resolution);
+    type = type.unaliased;
     registerBackendInstantiation(backend.compiler.boolClass, registry);
     bool inCheckedMode = backend.compiler.enableTypeAssertions;
     if (inCheckedMode) {