Use third_party packages for building try dart.

site/try/build_try.gyp

Index: site/try/build_try.gyp
diff --git a/site/try/build_try.gyp b/site/try/build_try.gyp
index c003fb0e93c64b74afcfe98ef622cda89bda17aa..9fa4b7cf274094be489062cb8046d0e35ac5035c 100644
--- a/site/try/build_try.gyp
+++ b/site/try/build_try.gyp
@@ -50,16 +50,16 @@
           '../../pkg/args/lib',
           '../../pkg/collection/lib',
           '../../pkg/crypto/lib',
-          '../../pkg/http/lib',
-          '../../pkg/http_parser/lib',
           '../../pkg/intl/lib',
           '../../pkg/logging/lib',
           '../../pkg/matcher/lib',
-          '../../pkg/path/lib',
-          '../../pkg/stack_trace/lib',
-          '../../pkg/string_scanner/lib',
           '../../pkg/unittest/lib',
-          '../../pkg/yaml/lib',
+          '../../third_party/pkg/http/lib',
+          '../../third_party/pkg/http_parser/lib',
+          '../../third_party/pkg/path/lib',
+          '../../third_party/pkg/stack_trace/lib',
+          '../../third_party/pkg/string_scanner/lib',
+          '../../third_party/pkg/yaml/lib',
         ],
       },
       'actions': [