Use --package-root when call build_sdk_summaries.dart script.

utils/dartanalyzer/dartanalyzer.gyp

 # 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.
 
 {
   'targets': [
     {
       'target_name': 'dartanalyzer',
       'type': 'none',
       'dependencies': [
@@ skipping 26 matching lines @@
             '../../sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart',
             '<(SHARED_INTERMEDIATE_DIR)/packages.stamp',
             '<!@(["python", "../../tools/list_files.py", "\\.dart$", "../../pkg/analyzer"])',
           ],
           'outputs': [
             '<(SHARED_INTERMEDIATE_DIR)/spec.sum',
             '<(SHARED_INTERMEDIATE_DIR)/strong.sum',
           ],
           'action': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
+            '--package-root=<(PRODUCT_DIR)/packages/',
             '../../pkg/analyzer/tool/summary/build_sdk_summaries.dart',
             '<(SHARED_INTERMEDIATE_DIR)',
           ],
         },
       ],
     },
   ],
 }