Add DartSdk.getLinkedBundle() and use it in PubSummaryManager.

pkg/analyzer/lib/src/dart/sdk/sdk.dart

Index: pkg/analyzer/lib/src/dart/sdk/sdk.dart
diff --git a/pkg/analyzer/lib/src/dart/sdk/sdk.dart b/pkg/analyzer/lib/src/dart/sdk/sdk.dart
index 82d5842afc60ec2c1f03b9b564e64962a564584c..453e6d113e3529eaaff9ce180563480064e0b3be 100644
--- a/pkg/analyzer/lib/src/dart/sdk/sdk.dart
+++ b/pkg/analyzer/lib/src/dart/sdk/sdk.dart
@@ -62,6 +62,8 @@ abstract class AbstractDartSdk implements DartSdk {
    */
   Map<String, Source> _uriToSourceMap = new HashMap<String, Source>();
 
+  PackageBundle _sdkBundle;
+
   /**
    * Set the [options] for this SDK analysis context.  Throw [StateError] if the
    * context has been already created.
@@ -82,7 +84,7 @@ abstract class AbstractDartSdk implements DartSdk {
       _analysisContext.sourceFactory = factory;
       if (_useSummary) {
         bool strongMode = _analysisOptions?.strongMode ?? false;
-        PackageBundle sdkBundle = getSummarySdkBundle(strongMode);
+        PackageBundle sdkBundle = getLinkedBundle();
         if (sdkBundle != null) {
           _analysisContext.resultProvider = new SdkSummaryResultProvider(
               _analysisContext, sdkBundle, strongMode);
@@ -151,6 +153,13 @@ abstract class AbstractDartSdk implements DartSdk {
     return null;
   }
 
+  @override
+  PackageBundle getLinkedBundle() {
+    bool strongMode = _analysisOptions?.strongMode ?? false;
+    _sdkBundle ??= getSummarySdkBundle(strongMode);
+    return _sdkBundle;
+  }
+
   String getRelativePathFromFile(File file);
 
   @override
@@ -272,6 +281,9 @@ class EmbedderSdk extends AbstractDartSdk {
   Map<String, String> get urlMappings => _urlMappings;
 
   @override
+  PackageBundle getLinkedBundle() => null;
+
+  @override
   String getRelativePathFromFile(File file) => file.path;
 
   @override