Add _sdkext support back to analysis server

pkg/analysis_server/test/context_manager_test.dart

Index: pkg/analysis_server/test/context_manager_test.dart
diff --git a/pkg/analysis_server/test/context_manager_test.dart b/pkg/analysis_server/test/context_manager_test.dart
index c810386b6b35c8fed3c84f24cdfa1a8da21857d1..41a16073813b2c9e2a03332a388b02b0de3cf118 100644
--- a/pkg/analysis_server/test/context_manager_test.dart
+++ b/pkg/analysis_server/test/context_manager_test.dart
@@ -344,6 +344,42 @@ analyzer:
     expect(contexts[1].name, equals('/my/proj/lib'));
   }
 
+  test_sdk_ext_packagespec() async {

[Paul Berry, 2015/08/13 17:12:07]

This test verifies correct behavior of your changes to PackagesFileDisposition.

It would be nice to also have a test that verifies that the new code in
PackageMapDisposition works, but unfortunately it's hard to write such a test
today because of dartbug.com/23909.  Would you mind adding a TODO comment
referencing bug 23909, so that when I get around to fixing that bug, I'll
remember to write the additional test?

[Cutch, 2015/08/13 19:24:17]

Done.

+    // Create files.
+    String libPath = newFolder([projPath, LIB_NAME]);
+    newFile([libPath, 'main.dart']);
+    newFile([libPath, 'nope.dart']);
+    String sdkExtPath = newFolder([projPath, 'sdk_ext']);
+    newFile([sdkExtPath, 'entry.dart']);
+    String sdkExtSrcPath = newFolder([projPath, 'sdk_ext', 'src']);
+    newFile([sdkExtSrcPath, 'part.dart']);
+    // Setup sdk extension mapping.
+    newFile(
+        [libPath, '_sdkext'],
+        r'''
+{
+  "dart:foobar": "../sdk_ext/entry.dart"
+}
+''');
+    // Setup .packages file
+    newFile(
+      [projPath, '.packages'],
+      r'''
+test_pack:lib/
+''');
+    // Setup context.
+    manager.setRoots(<String>[projPath], <String>[], <String, String>{});
+    // Confirm that one context was created.
+    var contexts =
+        manager.contextsInAnalysisRoot(resourceProvider.newFolder(projPath));
+    expect(contexts, isNotNull);
+    expect(contexts.length, equals(1));
+    var context = contexts[0];
+    var source = context.sourceFactory.forUri('dart:foobar');
+    expect(source.fullName, equals('/my/proj/sdk_ext/entry.dart'));
+  }
+
+
   test_refresh_folder_with_packagespec() {
     // create a context with a .packages file
     String packagespecFile = posix.join(projPath, '.packages');