Take Condition for async nesting expectations

pkgs/checks/CHANGELOG.md

@@ -4,6 +4,15 @@
     -   `checkThat` renamed to `check`.
     -   `nest` and `nestAsync` take `Iterable<String> Function()` arguments for
         `label` instead of `String`.
+    -   Async expectation extensions `completes`, `throws`, `emits`, and
+        `emitsError` no longer return a `Future<Subject>`. Instead they take an
+        optional `Condition` argument which can check expectations that would
+        have been checked on the returned subject.
+    -   `nestAsync` no longer returns a `Subject`, callers must pass the
+        followup `Condition` to the nullable argument.
+    -   Remove the `which` extension on `Future<Subject>`.
+-   Add a constructor for `Condition` which takes a callback to invoke when
+    `apply` or `applyAsync` is called.
 -   Added an example.
 -   Include a stack trace in the failure description for unexpected errors from
     Futures or Streams.

pkgs/checks/README.md

@@ -50,9 +50,8 @@ condition. The `it()` utility returns a `ConditionSubject`.
 check(someList).any(it()..isGreaterThan(0));
 ```
 
-Some complicated checks may be difficult to write with parenthesized awaited
-expressions, or impossible to write with cascade syntax. There are `which`
-utilities for both use cases which take a `Condition`.
+Some complicated checks may be not be possible to write with cascade syntax.
+There is a `which` utility for this use case which takes a `Condition`.
 
 ```dart
 check(someString)
@@ -61,10 +60,6 @@ check(someString)
   ..length.which(it()
     ..isGreatherThan(10)
     ..isLessThan(100));
-
-await check(someFuture)
-    .completes()
-    .which(it()..equals(expectedCompletion));
 ```
 
 # Writing custom expectations

pkgs/checks/lib/checks.dart

@@ -4,7 +4,7 @@
 
 export 'src/checks.dart' show check, Subject, Skip, it;
 export 'src/extensions/async.dart'
-    show ChainAsync, FutureChecks, StreamChecks, StreamQueueWrap;
+    show FutureChecks, StreamChecks, StreamQueueWrap;
 export 'src/extensions/core.dart'
     show BoolChecks, CoreChecks, NullabilityChecks;
 export 'src/extensions/function.dart' show ThrowsCheck;

pkgs/checks/lib/context.dart

@@ -4,14 +4,15 @@
 
 export 'src/checks.dart'
     show
-        Subject,
         CheckFailure,
         Condition,
+        ConditionSubject,
         Context,
         ContextExtension,
         Extracted,
         FailureDetail,
         Rejection,
+        Subject,
         describe,
         describeAsync,
         softCheck,