pkg/docgen: support better const rendering of String, List, and empty Map

pkg/docgen/test/multi_library_code/lib/test_lib.dart

 // Copyright (c) 2014, 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.
 
 library test_lib;
 
 import 'test_lib_foo.dart';
 import 'test_lib_bar.dart';
 export 'test_lib_foo.dart';
 export 'test_lib_bar.dart';
@@ skipping 21 matching lines @@
    */
   void doThis(int A) {
     print(A);
   }
 }
 
 // A trivial use of `B` and `C` to eliminate import warnings
 B sampleMethod(C cInstance) {
   throw new UnimplementedError();
 }
+
+int positionalDefaultValues([int intConst = INT_CONST,
+    bool boolConst = BOOL_CONST, List listConst = LIST_CONST,
+    String stringConst = STRING_CONST, Map mapConst = MAP_CONST,
+    Map emptyMap = EMPTY_MAP_CONST]) {
+  throw new UnimplementedError();
+}
+
+const int INT_CONST = 42;
+
+const bool BOOL_CONST = true;
+
+const LIST_CONST = const [true, 42, 'Shanna', null, 3.14, const []];
+
+const STRING_CONST = 'Shanna';
+
+const MAP_CONST = const {'a':1, 2: true, 'c': const [1,null,true]};
+
+const EMPTY_MAP_CONST = const {};