Use Feature directly in resolution.

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 6c3c9ca138e40585f93e14b60504cb57b7c4353a..65b84697c4ca31e21d046597bdf2b4239b3d856a 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -2743,6 +2743,9 @@ class JavaScriptImpactTransformer extends ImpactTransformer {
         case Feature.STRING_INTERPOLATION:
           registerBackendImpact(transformed, impacts.stringInterpolation);
           break;
+        case Feature.STRING_JUXTAPOSITION:
+          registerBackendImpact(transformed, impacts.stringJuxtaposition);
+          break;
         case Feature.SUPER_NO_SUCH_METHOD:
           registerBackendImpact(transformed, impacts.superNoSuchMethod);
           break;
@@ -2856,6 +2859,29 @@ class JavaScriptImpactTransformer extends ImpactTransformer {
       }
     }
 
+    for (ConstantExpression constant in worldImpact.constantLiterals) {
+      switch (constant.kind) {
+        case ConstantExpressionKind.NULL:
+          registerBackendImpact(transformed, impacts.nullLiteral);
+          break;
+        case ConstantExpressionKind.BOOL:
+          registerBackendImpact(transformed, impacts.boolLiteral);
+          break;
+        case ConstantExpressionKind.INT:
+          registerBackendImpact(transformed, impacts.intLiteral);
+          break;
+        case ConstantExpressionKind.DOUBLE:
+          registerBackendImpact(transformed, impacts.doubleLiteral);
+          break;
+        case ConstantExpressionKind.STRING:
+          registerBackendImpact(transformed, impacts.stringLiteral);
+          break;
+        default:
+          assert(invariant(NO_LOCATION_SPANNABLE, false,
+              message: "Unexpected constant literal: ${constant.kind}."));
+      }
+    }
+
     return transformed;
   }