Add Result.flatten.

pkg/async/lib/result.dart

Index: pkg/async/lib/result.dart
diff --git a/pkg/async/lib/result.dart b/pkg/async/lib/result.dart
index f76481c2d3a42afc621895442d8b844b2fb678a6..903c8a538aa8417901353ca6d6b1a5cf89f09901 100644
--- a/pkg/async/lib/result.dart
+++ b/pkg/async/lib/result.dart
@@ -27,7 +27,7 @@ abstract class Result<T> {
    * calling `computation`, or an [ErrorResult] with an error thrown by
    * the call.
    */
-  Result(T computation()) {
+  factory Result(T computation()) {
     try {
       return new ValueResult(computation());
     } catch (e, s) {
@@ -71,7 +71,7 @@ abstract class Result<T> {
   /**
    * Release the result of a captured future.
    *
-   * Converts the [Result] value of the given [future] to a result or error
+   * Converts the [Result] value of the given [future] to a value or error
    * completion of the returned future.
    *
    * If [future] completes with an error, the returned future completes with
@@ -107,6 +107,19 @@ abstract class Result<T> {
   }
 
   /**
+   * Converts a result of a result to a single result.
+   *
+   * If the result is an error, or it is a `Result` value
+   * which is then an error, then a result with that error is returned.
+   * Otherwise both levels of results are value results, and a single
+   * result with the value is returned.
+   */
+  static Result flatten(Result<Result> result) {
+    if (result.isError) return result;
+    return result.asValue.value;
+  }
+
+  /**
    * Whether this result is a value result.
    *
    * Always the opposite of [isError].