Complete with null, not return null as a Future.

pkg/analyzer/lib/src/dart/analysis/driver.dart

Index: pkg/analyzer/lib/src/dart/analysis/driver.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index 5e71f2059acb6ab96b1360f8e716807ff3a062b0..203944d3dfb316293a6d36d0e063abb0cbde939a 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -505,7 +505,7 @@ class AnalysisDriver {
    */
   Future<AnalysisDriverUnitIndex> getIndex(String path) {
     if (!_fsState.hasUri(path)) {
-      return null;
+      return new Future.value();
     }
     var completer = new Completer<AnalysisDriverUnitIndex>();
     _indexRequestedFiles
@@ -535,7 +535,7 @@ class AnalysisDriver {
    */
   Future<AnalysisResult> getResult(String path) {
     if (!_fsState.hasUri(path)) {
-      return null;
+      return new Future.value();
     }
 
     // Return the cached result.
@@ -575,7 +575,7 @@ class AnalysisDriver {
    */
   Future<CompilationUnitElement> getUnitElement(String path) {
     if (!_fsState.hasUri(path)) {
-      return null;
+      return new Future.value();
     }
     var completer = new Completer<CompilationUnitElement>();
     _unitElementRequestedFiles