Redo "Construct literal maps using factory constructor."

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index d2a7cd1d7c0234c9a744f4b857c9e83990da0417..f0b586fb33aebb67cdac3bf5a87f296bd14c5d30 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -61,7 +61,10 @@ const String DEFAULT_HELPERLIB = r'''
   class Closure implements Function {}
   class Null {}
   class Dynamic_ {}
-  class LinkedHashMap {}
+  class LinkedHashMap {
+    factory LinkedHashMap._empty() => null;
+    factory LinkedHashMap._literal(elements) => null;
+  }
   class ConstantMap {}
   class TypeImpl {}
   S() {}