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

tests/compiler/dart2js/least_upper_bound_test.dart

Index: tests/compiler/dart2js/least_upper_bound_test.dart
diff --git a/tests/compiler/dart2js/least_upper_bound_test.dart b/tests/compiler/dart2js/least_upper_bound_test.dart
index 9d7af4d23e13ab56e8632c52b500d96ab1fdd456..c4706bff5d4eb2fdeb655e88e7f7782211e4a7a9 100644
--- a/tests/compiler/dart2js/least_upper_bound_test.dart
+++ b/tests/compiler/dart2js/least_upper_bound_test.dart
@@ -381,12 +381,12 @@ void testFunction() {
     checkLub(DartType a, DartType b, DartType expectedLub) {
       DartType lub = env.computeLeastUpperBound(a, b);
       if (a != b) {
-        expectedLub = expectedLub.unalias(env.compiler.resolution);
-        lub = lub.unalias(env.compiler.resolution);
+        expectedLub = expectedLub.unaliased;
+        lub = lub.unaliased;
       }
       Expect.equals(expectedLub, lub,
-          'Unexpected lub(${a.unalias(env.compiler.resolution)},'
-                         '${b.unalias(env.compiler.resolution)}) = '
+          'Unexpected lub(${a.unaliased},'
+                         '${b.unaliased} = '
                          '${lub}, expected ${expectedLub}');
     }