Use SdkExtensionFinder in the old Analysis Server implementation.

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index 86b13a101c120425708e6795a0a7d54a5e83ce34..fd35213fc99a8963a0232387f8e2dd7d3d4b8de6 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -1710,6 +1710,10 @@ class CustomPackageResolverDisposition extends FolderDisposition {
   @override
   EmbedderYamlLocator getEmbedderLocator(ResourceProvider resourceProvider) =>
       new EmbedderYamlLocator(null);
+
+  @override
+  SdkExtensionFinder getSdkExtensionFinder(ResourceProvider resourceProvider) =>
+      new SdkExtensionFinder(null);
 }
 
 /**
@@ -1757,6 +1761,13 @@ abstract class FolderDisposition {
    * where that is necessary.
    */
   EmbedderYamlLocator getEmbedderLocator(ResourceProvider resourceProvider);
+
+  /**
+   * Return the extension finder used to locate the `_sdkext` file used to add
+   * extensions to the SDK. The [resourceProvider] is used to access the file
+   * system in cases where that is necessary.
+   */
+  SdkExtensionFinder getSdkExtensionFinder(ResourceProvider resourceProvider);
 }
 
 /**
@@ -1780,6 +1791,10 @@ class NoPackageFolderDisposition extends FolderDisposition {
   @override
   EmbedderYamlLocator getEmbedderLocator(ResourceProvider resourceProvider) =>
       new EmbedderYamlLocator(null);
+
+  @override
+  SdkExtensionFinder getSdkExtensionFinder(ResourceProvider resourceProvider) =>
+      new SdkExtensionFinder(null);
 }
 
 /**
@@ -1790,6 +1805,7 @@ class PackageMapDisposition extends FolderDisposition {
   final Map<String, List<Folder>> packageMap;
 
   EmbedderYamlLocator _embedderLocator;
+  SdkExtensionFinder _sdkExtensionFinder;
 
   @override
   final String packageRoot;
@@ -1814,6 +1830,11 @@ class PackageMapDisposition extends FolderDisposition {
     }
     return _embedderLocator;
   }
+
+  @override
+  SdkExtensionFinder getSdkExtensionFinder(ResourceProvider resourceProvider) {
+    return _sdkExtensionFinder ??= new SdkExtensionFinder(packageMap);
+  }
 }
 
 /**
@@ -1827,6 +1848,7 @@ class PackagesFileDisposition extends FolderDisposition {
   Map<String, List<Folder>> packageMap;
 
   EmbedderYamlLocator _embedderLocator;
+  SdkExtensionFinder _sdkExtensionFinder;
 
   PackagesFileDisposition(this.packages);
 
@@ -1868,4 +1890,10 @@ class PackagesFileDisposition extends FolderDisposition {
     }
     return _embedderLocator;
   }
+
+  @override
+  SdkExtensionFinder getSdkExtensionFinder(ResourceProvider resourceProvider) {
+    return _sdkExtensionFinder ??=
+        new SdkExtensionFinder(buildPackageMap(resourceProvider));
+  }
 }