Streamline AnalysisDriver status tracking.

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

Index: pkg/analyzer/lib/src/dart/analysis/status.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/status.dart b/pkg/analyzer/lib/src/dart/analysis/status.dart
index e70de686623aa72f3eafdff29498bc62f44a741c..b108eafcb8c81fd57bd6d78d84a24eb682290fa0 100644
--- a/pkg/analyzer/lib/src/dart/analysis/status.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/status.dart
@@ -89,10 +89,22 @@ class StatusSupport {
   Stream<AnalysisStatus> get stream => _statusController.stream;
 
   /**
+   * Prepare for the driver to start analyzing, but do not notify the [stream]
+   * yet.
+   *
+   * A call to [preTransitionToAnalyzing] has the same effect on [waitForIdle]
+   * as a call to [transitionToAnalyzing], but it has no effect on the [stream].
+   */
+  void preTransitionToAnalyzing() {
+    _idleCompleter ??= new Completer<Null>();
+  }
+
+  /**
    * Send a notifications to the [stream] that the driver started analyzing.

[scheglov, 2017/02/01 00:35:58]

Now probably that "a driver" started analyzing.
Or maybe don't mention drivers here and elsewhere in this class.

[Paul Berry, 2017/02/01 17:17:34]

Done.

    */
   void transitionToAnalyzing() {
     if (_currentStatus != AnalysisStatus.ANALYZING) {
+      preTransitionToAnalyzing();
       _currentStatus = AnalysisStatus.ANALYZING;
       _statusController.add(AnalysisStatus.ANALYZING);
     }
@@ -105,9 +117,9 @@ class StatusSupport {
     if (_currentStatus != AnalysisStatus.IDLE) {
       _currentStatus = AnalysisStatus.IDLE;
       _statusController.add(AnalysisStatus.IDLE);
-      _idleCompleter?.complete();
-      _idleCompleter = null;
     }
+    _idleCompleter?.complete();
+    _idleCompleter = null;
   }
 
   /**
@@ -117,11 +129,6 @@ class StatusSupport {
    * immediately.
    */
   Future<Null> waitForIdle() {
-    if (_currentStatus == AnalysisStatus.IDLE) {
-      return new Future.value();
-    } else {
-      _idleCompleter ??= new Completer<Null>();
-      return _idleCompleter.future;
-    }
+    return _idleCompleter?.future ?? new Future.value();
   }
 }