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

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 503 matching lines @@
       return constantCompiler.compileConstant(element);
     });
   }
 
   void compileVariable(VariableElement element) {
     measure(() {
       constantCompiler.compileVariable(element);
     });
   }
 
-  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);
     });
   }
 
   ConstantExpression compileMetadata(MetadataAnnotation metadata,
                            Node node,
                            TreeElements elements) {
     return measure(() {
       return constantCompiler.compileMetadata(metadata, node, elements);
     });
   }
@@ skipping 19 matching lines @@
   }
 
   void traceGraph(String title, var irObject) {
     compiler.tracer.traceGraph(title, irObject);
   }
 
   DartTypes get dartTypes => compiler.types;
 
   InternalErrorFunction get internalError => compiler.internalError;
 }