Allow use of deferred type-literals in non-constant contexts.

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

Index: pkg/compiler/lib/src/dart_backend/backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend.dart b/pkg/compiler/lib/src/dart_backend/backend.dart
index 763e2e3c95f22e7619367b5f662a15414dcef821..90ae82eebdc1ae985b2a566c9b82c4f30b5c722c 100644
--- a/pkg/compiler/lib/src/dart_backend/backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend.dart
@@ -521,9 +521,11 @@ class DartConstantTask extends ConstantCompilerTask
     });
   }
 
-  ConstantExpression compileNode(Node node, TreeElements elements) {
+  ConstantExpression compileNode(Node node, TreeElements elements,
+                                 {bool enforceConst: true}) {
     return measure(() {
-      return constantCompiler.compileNodeWithDefinitions(node, elements);
+      return constantCompiler.compileNodeWithDefinitions(node, elements,
+          isConst: enforceConst);
     });
   }