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

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

Index: pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart b/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
index beb05de1da9dfeb3434ff3d9e21128967290515f..a578d2efb1f494c2378aaa9bbc9b60ca5456f979 100644
--- a/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend_ast_emitter.dart
@@ -1279,6 +1279,12 @@ class ConstantEmitter
                         BuilderContext<Statement> context) {
     return handlePrimitiveConstant(exp.value);
   }
+
+  @override
+  Expression visitDeferred(DeferredConstantExpression exp,
+                           BuilderContext<Statement> context) {
+    return exp.expression.accept(this);
+  }
 }
 
 /// Moves function parameters into a separate variable if one of its uses is