Support for overlay only files in Analysis Server with the new driver.

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 8bbadccbfe05d26f957784f79d4fb3257ccb1f6e..5f4460ca3e5f6f8e2f437c4b729bdc4b67ed126c 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -279,6 +279,17 @@ abstract class ContextManager {
   List<AnalysisContext> contextsInAnalysisRoot(Folder analysisRoot);
 
   /**
+   * Return the [AnalysisDriver] for the "innermost" context whose associated
+   * folder is or contains the given path.  ("innermost" refers to the nesting
+   * of contexts, so if there is a context for path /foo and a context for
+   * path /foo/bar, then the innermost context containing /foo/bar/baz.dart is
+   * the context for /foo/bar.)
+   *
+   * If no driver contains the given path, `null` is returned.
+   */
+  AnalysisDriver getDriverFor(String path);
+
+  /**
    * Return the [AnalysisContext] for the "innermost" context whose associated
    * folder is or contains the given path.  ("innermost" refers to the nesting
    * of contexts, so if there is a context for path /foo and a context for
@@ -583,6 +594,11 @@ class ContextManagerImpl implements ContextManager {
   bool definesEmbeddedLibs(Map map) => map[_EMBEDDED_LIB_MAP_KEY] != null;
 
   @override
+  AnalysisDriver getDriverFor(String path) {
+    return _getInnermostContextInfoFor(path)?.analysisDriver;
+  }
+
+  @override
   AnalysisContext getContextFor(String path) {
     return _getInnermostContextInfoFor(path)?.context;
   }