Build strong mode SDK summary with dart2js paths

pkg/analyzer/tool/summary/build_sdk_summaries.dart

 import 'dart:io';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_io.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/sdk_io.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/summary/flat_buffers.dart' as fb;
@@ skipping 165 matching lines @@
    * Build a strong or spec mode summary for the Dart SDK at [sdkPath].
    */
   _BuilderOutput build() {
     String modeName = strongMode ? 'strong' : 'spec';
     print('Generating $modeName mode summary and index.');
     Stopwatch sw = new Stopwatch()..start();
     //
     // Prepare SDK.
     //
     DirectoryBasedDartSdk sdk =
-        new DirectoryBasedDartSdk(new JavaFile(sdkPath));
+        new DirectoryBasedDartSdk(new JavaFile(sdkPath), strongMode);
     sdk.useSummary = false;
     sdk.analysisOptions = new AnalysisOptionsImpl()..strongMode = strongMode;
     context = sdk.context;
     //
     // Prepare 'dart:' URIs to serialize.
     //
     Set<String> uriSet =
         sdk.sdkLibraries.map((SdkLibrary library) => library.shortName).toSet();
-    uriSet.add('dart:html/nativewrappers.dart');
+    if (!strongMode) {
+      uriSet.add('dart:html/nativewrappers.dart');
+    }
     uriSet.add('dart:html_common/html_common_dart2js.dart');
     //
     // Serialize each SDK library.
     //
     for (String uri in uriSet) {
       Source libSource = sdk.mapDartUri(uri);
       _serializeLibrary(libSource);
     }
     //
     // Assemble the output.
@@ skipping 47 matching lines @@
     }
   }
 }
 
 class _Output {
   final _BuilderOutput spec;
   final _BuilderOutput strong;
 
   _Output(this.spec, this.strong);
 }