backoff: add reset method to reset to minimal wait

Author: chrisirhc

lib/api/backoff.dart

@@ -49,4 +49,10 @@ class BackoffMachine {
 
     _waitsCompleted++;
   }
+
+  /// Reset the machine to its initial state with minimal wait.
+  void reset() {
+    _startTime = null;
+    _waitsCompleted = 0;
+  }
 }

test/api/backoff_test.dart

@@ -51,4 +51,17 @@ void main() {
       check(maxFromAllTrials).isGreaterThan(expectedMax * 0.75);
     }
   });
+
+  test('BackoffMachine resets duration', () async {
+    final backoffMachine = BackoffMachine();
+    await measureWait(backoffMachine.wait());
+    final duration2 = await measureWait(backoffMachine.wait());
+    check(backoffMachine.waitsCompleted).equals(2);
+
+    backoffMachine.reset();
+    check(backoffMachine.waitsCompleted).equals(0);
+    final durationReset1 = await measureWait(backoffMachine.wait());
+    check(durationReset1).isLessThan(duration2);
+    check(backoffMachine.waitsCompleted).equals(1);
+  });
 }