diff --git a/pkg/async_helper/lib/async_helper.dart b/pkg/async_helper/lib/async_helper.dart
index 1f3acb1b329..d4d536f5a56 100644
--- a/pkg/async_helper/lib/async_helper.dart
+++ b/pkg/async_helper/lib/async_helper.dart
@@ -89,56 +89,50 @@ Future<void> asyncTest(f()) {
   return f().then(asyncSuccess);
 }
 
-bool _pass(dynamic object) => true;
-
-/// Calls [f] and verifies that it throws a `T`.
+/// Verifies that the asyncronous [result] throws a [T].
 ///
-/// The optional [check] function can provide additional validation that the
-/// correct object is being thrown. For example, to check the content of the
-/// thrown object you could write this:
+/// Fails if [result] completes with a value, or it completes with
+/// an error which is not a [T].
 ///
-///     asyncExpectThrows<MyException>(myThrowingFunction,
-///          (e) => e.myMessage.contains("WARNING"));
-///
-/// If `f` fails an expectation (i.e., throws an [ExpectException]), that
-/// exception is not caught by [asyncExpectThrows]. The test is still considered
-/// failing.
-void asyncExpectThrows<T>(Future<void> f(),
-    [bool check(T error) = _pass, String reason = ""]) {
-  var type = "";
-  if (T != dynamic && T != Object) type = "<$T>";
+/// Returns the accepted thrown object.
+/// For example, to check the content of the thrown object,
+/// you could write this:
+/// ```
+/// var e = await asyncExpectThrows<MyException>(asyncExpression)
+/// Expect.isTrue(e.myMessage.contains("WARNING"));
+/// ```
+/// If `result` completes with an [ExpectException] error from another
+/// failed test expectation, that error cannot be caught and accepted.
+Future<T> asyncExpectThrows<T extends Object>(Future<void> result,
+    [String reason = ""]) {
   // Handle null being passed in from legacy code while also avoiding producing
   // an unnecessary null check warning here.
   if ((reason as dynamic) == null) reason = "";
+
+  var type = "";
+  if (T != dynamic && T != Object) type = "<$T>";
   var header = "asyncExpectThrows$type(${reason}):";
 
+  if ((result as dynamic) == null) {
+    Expect.testError("$header result Future must not be null.");
+  }
   // TODO(rnystrom): It might useful to validate that T is not bound to
   // ExpectException since that won't work.
 
-  if (f is! Function()) {
-    // Only throws from executing the function body should count as throwing.
-    // The failure to even call `f` should throw outside the try/catch.
-    Expect.testError("$header Function not callable with zero arguments.");
-  }
-
-  var result = f();
-  if (result is! Future) {
-    Expect.testError("$header Function did not return a Future.");
-  }
-
   asyncStart();
-  result.then<Null>((_) {
+  return result.then<T>((_) {
     throw ExpectException("$header Did not throw.");
-  }).catchError((error, stack) {
+  }, onError: (error, stack) {
     // A test failure doesn't count as throwing.
     if (error is ExpectException) throw error;
 
-    if (error is! T || (check != null && !check(error))) {
+    if (error is! T) {
       // Throws something unexpected.
       throw ExpectException(
           "$header Unexpected '${Error.safeToString(error)}'\n$stack");
     }
 
     asyncEnd();
+    return error;
   });
 }
diff --git a/pkg/async_helper/lib/async_minitest.dart b/pkg/async_helper/lib/async_minitest.dart
index fa5d319cf02..5b58930c8f6 100644
--- a/pkg/async_helper/lib/async_minitest.dart
+++ b/pkg/async_helper/lib/async_minitest.dart
@@ -215,14 +215,14 @@ void isNull(dynamic o) {
   Expect.isNull(o);
 }
 
-void _checkThrow<T>(dynamic v, void onError(error)) {
+void _checkThrow<T extends Object>(dynamic v, void onError(error)) {
   if (v is Future) {
     asyncStart();
     v.then((_) {
       Expect.fail("Did not throw");
     }, onError: (e, s) {
       if (e is! T) throw e;
-      if (onError != null) onError(e);
+      onError(e);
       asyncEnd();
     });
     return;
diff --git a/pkg/expect/lib/expect.dart b/pkg/expect/lib/expect.dart
index 1b1a534ade5..e7413dd1731 100644
--- a/pkg/expect/lib/expect.dart
+++ b/pkg/expect/lib/expect.dart
@@ -243,9 +243,13 @@ class Expect {
         buffer.write('_');
       } else {
         int first = equivalence[0];
-        buffer..write('#')..write(first);
+        buffer
+          ..write('#')
+          ..write(first);
         if (first == i) {
-          buffer..write('=')..write(objects[i]);
+          buffer
+            ..write('=')
+            ..write(objects[i]);
         }
       }
     }
@@ -259,7 +263,10 @@ class Expect {
     if (first.isNotEmpty && first.length == objects.length) return;
     var buffer = new StringBuffer("Expect.allIdentical([");
     _writeEquivalences(objects, equivalences, buffer);
-    buffer..write("]")..write(msg)..write(")");
+    buffer
+      ..write("]")
+      ..write(msg)
+      ..write(")");
     _fail(buffer.toString());
   }
 
@@ -290,7 +297,10 @@ class Expect {
     if (!hasEquivalence) return;
     var buffer = new StringBuffer("Expect.allDistinct([");
     _writeEquivalences(objects, equivalences, buffer);
-    buffer..write("]")..write(msg)..write(")");
+    buffer
+      ..write("]")
+      ..write(msg)
+      ..write(")");
     _fail(buffer.toString());
   }
 
@@ -387,7 +397,7 @@ class Expect {
     String defaultMessage =
         'Expect.stringEquals(expected: <$expected>", <$actual>$msg) fails';
 
-    if ((expected == null) || (actual == null)) {
+    if ((expected as dynamic) == null || (actual as dynamic) == null) {
       _fail('$defaultMessage');
     }
 
@@ -583,45 +593,52 @@ class Expect {
     }
   }
 
-  static bool _defaultCheck([dynamic e]) => true;
+  static bool _defaultCheck([dynamic _]) => true;
 
   /**
-   * Calls the function [f] and verifies that it throws a `T`.
-   * The optional [check] function can provide additional validation
-   * that the correct object is being thrown.  For example, to check
-   * the content of the thrown boject you could write this:
+   * Verifies that [computation] throws a [T].
    *
-   *     Expect.throws<MyException>(myThrowingFunction,
-   *          (e) => e.myMessage.contains("WARNING"));
+   * Calls the [computation] function and fails if that call doesn't throw,
+   * throws something which is not a [T], or throws a [T] which does not
+   * satisfy the optional [check] function.
    *
-   * The type variable can be omitted and the type checked in [check]
-   * instead. This was traditionally done before Dart had generic methods.
+   * Returns the accepted thrown [T] object, if one is caught.
+   * This value can be checked further, instead of checking it in the [check]
+   * function. For example, to check the content of the thrown object,
+   * you could write this:
+   * ```
+   * var e = Expect.throws<MyException>(myThrowingFunction);
+   * Expect.isTrue(e.myMessage.contains("WARNING"));
+   * ```
+   * The type variable can be omitted, in which case it defaults to [Object],
+   * and the (sub-)type of the object can be checked in [check] instead.
+   * This was traditionally done before Dart had generic methods.
    *
-   * If `f` fails an expectation (i.e., throws an [ExpectException]), that
-   * exception is not caught by [Expect.throws]. The test is still considered
-   * failing.
+   * If `computation` fails another test expectation
+   * (i.e., throws an [ExpectException]),
+   * that exception cannot be caught and accepted by [Expect.throws].
+   * The test is still considered failing.
    */
-  static void throws<T>(void f(),
-      [bool check(T error) = _defaultCheck, String reason = ""]) {
+  static T throws<T extends Object>(void Function() computation,
+      [bool Function(T error)? check, String? reason]) {
     // TODO(vsm): Make check and reason nullable or change call sites.
     // Existing tests pass null to set a reason and/or pass them through
     // via helpers.
     // TODO(rnystrom): Using the strange form below instead of "??=" to avoid
     // warnings of unnecessary null checks when analyzed as NNBD code.
-    if ((check as dynamic) == null) check = _defaultCheck;
-    if ((reason as dynamic) == null) reason = "";
+    reason ??= "";
     String msg = reason.isEmpty ? "" : "($reason)";
-    if (f is! Function()) {
+    if ((computation as dynamic) == null) {
       // Only throws from executing the function body should count as throwing.
       // The failure to even call `f` should throw outside the try/catch.
-      _fail("Expect.throws$msg: Function f not callable with zero arguments");
+      testError("Function must not be null");
     }
     try {
-      f();
+      computation();
     } catch (e, s) {
       // A test failure doesn't count as throwing.
       if (e is ExpectException) rethrow;
-      if (e is T && check(e as dynamic)) return;
+      if (e is T && (check ?? _defaultCheck)(e)) return e;
       // Throws something unexpected.
       String type = "";
       if (T != dynamic && T != Object) {
@@ -633,47 +650,41 @@ class Expect {
     _fail('Expect.throws$msg fails: Did not throw');
   }
 
-  static void throwsArgumentError(void f(), [String reason = "ArgumentError"]) {
-    Expect.throws(f, (error) => error is ArgumentError, reason);
-  }
+  static ArgumentError throwsArgumentError(void f(),
+          [String reason = "ArgumentError"]) =>
+      Expect.throws<ArgumentError>(f, _defaultCheck, reason);
 
-  static void throwsAssertionError(void f(),
-      [String reason = "AssertionError"]) {
-    Expect.throws(f, (error) => error is AssertionError, reason);
-  }
+  static AssertionError throwsAssertionError(void f(),
+          [String reason = "AssertionError"]) =>
+      Expect.throws<AssertionError>(f, _defaultCheck, reason);
 
-  static void throwsFormatException(void f(),
-      [String reason = "FormatException"]) {
-    Expect.throws(f, (error) => error is FormatException, reason);
-  }
+  static FormatException throwsFormatException(void f(),
+          [String reason = "FormatException"]) =>
+      Expect.throws<FormatException>(f, _defaultCheck, reason);
 
-  static void throwsNoSuchMethodError(void f(),
-      [String reason = "NoSuchMethodError"]) {
-    Expect.throws(f, (error) => error is NoSuchMethodError, reason);
-  }
+  static NoSuchMethodError throwsNoSuchMethodError(void f(),
+          [String reason = "NoSuchMethodError"]) =>
+      Expect.throws<NoSuchMethodError>(f, _defaultCheck, reason);
 
-  static void throwsReachabilityError(void f(),
-      [String reason = "ReachabilityError"]) {
-    Expect.throws(
-        f, (error) => error.toString().startsWith('ReachabilityError'), reason);
-  }
+  static Error throwsReachabilityError(void f(),
+          [String reason = "ReachabilityError"]) =>
+      Expect.throws<Error>(f,
+          (error) => error.toString().startsWith('ReachabilityError'), reason);
 
-  static void throwsRangeError(void f(), [String reason = "RangeError"]) {
-    Expect.throws(f, (error) => error is RangeError, reason);
-  }
+  static RangeError throwsRangeError(void f(),
+          [String reason = "RangeError"]) =>
+      Expect.throws<RangeError>(f, _defaultCheck, reason);
 
-  static void throwsStateError(void f(), [String reason = "StateError"]) {
-    Expect.throws(f, (error) => error is StateError, reason);
-  }
+  static StateError throwsStateError(void f(),
+          [String reason = "StateError"]) =>
+      Expect.throws<StateError>(f, _defaultCheck, reason);
 
-  static void throwsTypeError(void f(), [String reason = "TypeError"]) {
-    Expect.throws(f, (error) => error is TypeError, reason);
-  }
+  static TypeError throwsTypeError(void f(), [String reason = "TypeError"]) =>
+      Expect.throws<TypeError>(f, _defaultCheck, reason);
 
-  static void throwsUnsupportedError(void f(),
-      [String reason = "UnsupportedError"]) {
-    Expect.throws(f, (error) => error is UnsupportedError, reason);
-  }
+  static UnsupportedError throwsUnsupportedError(void f(),
+          [String reason = "UnsupportedError"]) =>
+      Expect.throws<UnsupportedError>(f, _defaultCheck, reason);
 
   /// Reports that there is an error in the test itself and not the code under
   /// test.
@@ -748,20 +759,20 @@ class ExpectException {
     _getTestName = getName;
   }
 
-  // TODO(rnystrom): Type this `String Function()?` once this library doesn't
-  // need to be NNBD-agnostic.
-  static dynamic _getTestName;
+  static String Function() _getTestName = _kEmptyString;
 
   final String message;
   final String name;
 
-  ExpectException(this.message)
-      : name = (_getTestName == null) ? "" : _getTestName();
+  ExpectException(this.message) : name = _getTestName();
 
   String toString() {
     if (name != "") return 'In test "$name" $message';
     return message;
   }
+
+  /// Initial value for _getTestName.
+  static String _kEmptyString() => "";
 }
 
 /// Is true iff type assertions are enabled.
@@ -769,7 +780,7 @@ class ExpectException {
 final bool typeAssertionsEnabled = (() {
   try {
     dynamic i = 42;
-    String s = i;
+    String s = i; // ignore: unused_local_variable
   } on TypeError {
     return true;
   }
diff --git a/pkg/expect/test/nnbd_mode_test.dart b/pkg/expect/test/nnbd_mode_test.dart
index 35dd3451719..1bacdec2cc4 100644
--- a/pkg/expect/test/nnbd_mode_test.dart
+++ b/pkg/expect/test/nnbd_mode_test.dart
@@ -6,7 +6,7 @@ import "package:expect/expect.dart";
 
 final bool strong = () {
   try {
-    int i = null as dynamic;
+    int i = null as dynamic; // ignore: unused_local_variable
     return false;
   } catch (e) {
     return true;
diff --git a/pkg/vm/testcases/transformations/type_flow/transformer/write_only_field2.dart.expect b/pkg/vm/testcases/transformations/type_flow/transformer/write_only_field2.dart.expect
index 4203e7a2b68..ba91b27cf6b 100644
--- a/pkg/vm/testcases/transformations/type_flow/transformer/write_only_field2.dart.expect
+++ b/pkg/vm/testcases/transformations/type_flow/transformer/write_only_field2.dart.expect
@@ -56,7 +56,7 @@ static method main() → void {
   new self::A::•();
   new self::B::•();
   self::C<core::num> c = new self::D::•();
-  exp::Expect::throws<dynamic>(() → void {
+  exp::Expect::throws<core::Object>(() → void {
     [@vm.call-site-attributes.metadata=receiverType:#lib::C<dart.core::num>] [@vm.direct-call.metadata=#lib::D.bar] c.{self::C::bar} = 3.14;
   });
   self::E e = new self::F::•();
diff --git a/tests/language/control_flow_collections/await_for_dynamic_null_test.dart b/tests/language/control_flow_collections/await_for_dynamic_null_test.dart
index aad701f24e2..36286b9381b 100644
--- a/tests/language/control_flow_collections/await_for_dynamic_null_test.dart
+++ b/tests/language/control_flow_collections/await_for_dynamic_null_test.dart
@@ -9,11 +9,14 @@ void main() {
   asyncTest(() async {
     // Null stream.
     dynamic nullStream = null;
-    asyncExpectThrows<Error>(
-        () async => <int>[await for (var i in nullStream) 1]);
-    asyncExpectThrows<Error>(
-        () async => <int, int>{await for (var i in nullStream) 1: 1});
-    asyncExpectThrows<Error>(
-        () async => <int>{await for (var i in nullStream) 1});
+    asyncExpectThrows<Error>(() async {
+      <int>[await for (var i in nullStream) 1];
+    }());
+    asyncExpectThrows<Error>(() async {
+      <int, int>{await for (var i in nullStream) 1: 1};
+    }());
+    asyncExpectThrows<Error>(() async {
+      <int>{await for (var i in nullStream) 1};
+    }());
   });
 }
diff --git a/tests/language/control_flow_collections/await_for_test.dart b/tests/language/control_flow_collections/await_for_test.dart
index ce84fe082d5..d98f9f2c317 100644
--- a/tests/language/control_flow_collections/await_for_test.dart
+++ b/tests/language/control_flow_collections/await_for_test.dart
@@ -30,17 +30,28 @@ Future<void> testList() async {
   Expect.listEquals(list, <int>[await for (var i in stream(list)) i]);
 
   // Await for at beginning.
-  Expect.listEquals(list, <int>[await for (var i in stream([1, 2])) i, 3, 4]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([1, 2])) i,
+    3,
+    4
+  ]);
 
   // Await for in middle.
-  Expect.listEquals(list, <int>[1, await for (var i in stream([2, 3])) i, 4]);
+  Expect.listEquals(list, <int>[
+    1,
+    await for (var i in stream([2, 3])) i,
+    4
+  ]);
 
   // Await for at end.
-  Expect.listEquals(list, <int>[1, 2, await for (var i in stream([3, 4])) i]);
+  Expect.listEquals(list, <int>[
+    1,
+    2,
+    await for (var i in stream([3, 4])) i
+  ]);
 
   // Empty await for.
-  Expect.listEquals(list,
-      <int>[1, 2, await for (var i in stream([])) i, 3, 4]);
+  Expect.listEquals(list, <int>[1, 2, await for (var i in stream([])) i, 3, 4]);
 
   // Multiple await fors.
   Expect.listEquals(list, <int>[
@@ -50,26 +61,44 @@ Future<void> testList() async {
   ]);
 
   // Spread inside await for.
-  Expect.listEquals(list,
-      <int>[await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]
+  ]);
 
   // If inside await for.
-  Expect.listEquals(list,
-      <int>[await for (var i in stream([1, 9, 2, 3, 9, 4])) if (i != 9) i]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([1, 9, 2, 3, 9, 4]))
+      if (i != 9) i
+  ]);
 
   // Else inside await for.
-  Expect.listEquals(list,
-      <int>[await for (var i in stream([1, -2, 3, -4])) if (i < 0) -i else i]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([1, -2, 3, -4]))
+      if (i < 0) -i else i
+  ]);
 
   // For inside await for.
   Expect.listEquals(list, <int>[
-    await for (var i in stream([0, 2])) for (var j = 1; j <= 2; j++) i + j
+    await for (var i in stream([0, 2]))
+      for (var j = 1; j <= 2; j++) i + j
   ]);
 
   // Does not flatten nested collection literal.
-  Expect.listEquals([1], [await for (var i in stream([1])) [i]].first);
-  Expect.mapEquals({1: 1}, [await for (var i in stream([1])) {i: i}].first);
-  Expect.setEquals({1}, [await for (var i in stream([1])) {i}].first);
+  Expect.listEquals(
+      [1],
+      [
+        await for (var i in stream([1])) [i]
+      ].first);
+  Expect.mapEquals(
+      {1: 1},
+      [
+        await for (var i in stream([1])) {i: i}
+      ].first);
+  Expect.setEquals(
+      {1},
+      [
+        await for (var i in stream([1])) {i}
+      ].first);
 }
 
 Future<void> testMap() async {
@@ -77,26 +106,30 @@ Future<void> testMap() async {
   Expect.mapEquals(map, <int, int>{await for (var i in stream(list)) i: i});
 
   // Await for at beginning.
-  Expect.mapEquals(map,
-      <int, int>{await for (var i in stream([1, 2])) i: i, 3: 3, 4: 4});
-
-  // Await for in middle.
-  Expect.mapEquals(map,
-      <int, int>{1: 1, await for (var i in stream([2, 3])) i: i, 4: 4});
-
-  // Await for at end.
-  Expect.mapEquals(map,
-      <int, int>{1: 1, 2: 2, await for (var i in stream([3, 4])) i: i});
-
-  // Empty await for.
   Expect.mapEquals(map, <int, int>{
-    1: 1,
-    await for (var i in stream([])) i: i,
-    2: 2,
+    await for (var i in stream([1, 2])) i: i,
     3: 3,
     4: 4
   });
 
+  // Await for in middle.
+  Expect.mapEquals(map, <int, int>{
+    1: 1,
+    await for (var i in stream([2, 3])) i: i,
+    4: 4
+  });
+
+  // Await for at end.
+  Expect.mapEquals(map, <int, int>{
+    1: 1,
+    2: 2,
+    await for (var i in stream([3, 4])) i: i
+  });
+
+  // Empty await for.
+  Expect.mapEquals(map,
+      <int, int>{1: 1, await for (var i in stream([])) i: i, 2: 2, 3: 3, 4: 4});
+
   // Multiple await fors.
   Expect.mapEquals(map, <int, int>{
     await for (var i in stream([1])) i: i,
@@ -106,18 +139,22 @@ Future<void> testMap() async {
 
   // Spread inside await for.
   Expect.mapEquals(map, <int, int>{
-    await for (var i in stream([0, 2]))
-      ...<int, int>{1 + i: 1 + i, 2 + i: 2 + i}
+    await for (var i in stream([0, 2])) ...<int, int>{
+      1 + i: 1 + i,
+      2 + i: 2 + i
+    }
   });
 
   // If inside await for.
   Expect.mapEquals(map, <int, int>{
-    await for (var i in stream([1, 9, 2, 3, 9, 4])) if (i != 9) i: i
+    await for (var i in stream([1, 9, 2, 3, 9, 4]))
+      if (i != 9) i: i
   });
 
   // Else inside await for.
   Expect.mapEquals(map, <int, int>{
-    await for (var i in stream([1, -2, 3, -4])) if (i < 0) -i: -i else i: i
+    await for (var i in stream([1, -2, 3, -4]))
+      if (i < 0) -i: -i else i: i
   });
 
   // For inside await for.
@@ -132,17 +169,28 @@ Future<void> testSet() async {
   Expect.setEquals(set, <int>{await for (var i in stream(list)) i});
 
   // Await for at beginning.
-  Expect.setEquals(set, <int>{await for (var i in stream([1, 2])) i, 3, 4});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([1, 2])) i,
+    3,
+    4
+  });
 
   // Await for in middle.
-  Expect.setEquals(set, <int>{1, await for (var i in stream([2, 3])) i, 4});
+  Expect.setEquals(set, <int>{
+    1,
+    await for (var i in stream([2, 3])) i,
+    4
+  });
 
   // Await for at end.
-  Expect.setEquals(set, <int>{1, 2, await for (var i in stream([3, 4])) i});
+  Expect.setEquals(set, <int>{
+    1,
+    2,
+    await for (var i in stream([3, 4])) i
+  });
 
   // Empty await for.
-  Expect.setEquals(set,
-      <int>{1, await for (var i in stream([])) i, 2, 3, 4});
+  Expect.setEquals(set, <int>{1, await for (var i in stream([])) i, 2, 3, 4});
 
   // Multiple await fors.
   Expect.setEquals(set, <int>{
@@ -152,26 +200,44 @@ Future<void> testSet() async {
   });
 
   // Spread inside await for.
-  Expect.setEquals(set,
-      <int>{await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]
+  });
 
   // If inside await for.
-  Expect.setEquals(set,
-      <int>{await for (var i in stream([1, 9, 2, 3, 9, 4])) if (i != 9) i});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([1, 9, 2, 3, 9, 4]))
+      if (i != 9) i
+  });
 
   // Else inside await for.
-  Expect.setEquals(set,
-      <int>{await for (var i in stream([1, -2, 3, -4])) if (i < 0) -i else i});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([1, -2, 3, -4]))
+      if (i < 0) -i else i
+  });
 
   // For inside await for.
   Expect.setEquals(set, <int>{
-    await for (var i in stream([0, 2])) for (var j = 1; j <= 2; j++) i + j
+    await for (var i in stream([0, 2]))
+      for (var j = 1; j <= 2; j++) i + j
   });
 
   // Does not flatten nested collection literal.
-  Expect.listEquals([1], {await for (var i in stream([1])) [i]}.first);
-  Expect.mapEquals({1: 1}, {await for (var i in stream([1])) {i: i}}.first);
-  Expect.setEquals({1}, {await for (var i in stream([1])) {i}}.first);
+  Expect.listEquals(
+      [1],
+      {
+        await for (var i in stream([1])) [i]
+      }.first);
+  Expect.mapEquals(
+      {1: 1},
+      {
+        await for (var i in stream([1])) {i: i}
+      }.first);
+  Expect.setEquals(
+      {1},
+      {
+        await for (var i in stream([1])) {i}
+      }.first);
 }
 
 Future<void> testDuplicateKeys() async {
@@ -207,28 +273,46 @@ Future<void> testKeyOrder() async {
   Expect.equals("1:a,2:a", map.keys.join(","));
   Expect.equals("2,4", map.values.join(","));
 
-  var set = <Equality>{e1a, await for (var i in stream([0, 1, 2])) keys[i]};
+  var set = <Equality>{
+    e1a,
+    await for (var i in stream([0, 1, 2])) keys[i]
+  };
   Expect.equals("1:a,2:a", set.join(","));
 }
 
 Future<void> testRuntimeErrors() async {
   // Cast variable.
   dynamic nonStream = 3;
-  asyncExpectThrows<TypeError>(
-      () async => <int>[await for (int i in nonStream) 1]);
-  asyncExpectThrows<TypeError>(
-      () async => <int, int>{await for (int i in nonStream) 1: 1});
-  asyncExpectThrows<TypeError>(
-      () async => <int>{await for (int i in nonStream) 1});
+  asyncExpectThrows<TypeError>(() async {
+    <int>[await for (int i in nonStream) 1];
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int, int>{await for (int i in nonStream) 1: 1};
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int>{await for (int i in nonStream) 1};
+  }());
 
   // Wrong element type.
   dynamic nonInt = "string";
-  asyncExpectThrows<TypeError>(
-      () async => <int>[await for (var i in stream([1])) nonInt]);
-  asyncExpectThrows<TypeError>(
-      () async => <int, int>{await for (var i in stream([1])) nonInt: 1});
-  asyncExpectThrows<TypeError>(
-      () async => <int, int>{await for (var i in stream([1])) 1: nonInt});
-  asyncExpectThrows<TypeError>(
-      () async => <int>{await for (var i in stream([1])) nonInt});
+  asyncExpectThrows<TypeError>(() async {
+    <int>[
+      await for (var i in stream([1])) nonInt
+    ];
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int, int>{
+      await for (var i in stream([1])) nonInt: 1
+    };
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int, int>{
+      await for (var i in stream([1])) 1: nonInt
+    };
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int>{
+      await for (var i in stream([1])) nonInt
+    };
+  }());
 }
diff --git a/tests/language_2/control_flow_collections/await_for_null_test.dart b/tests/language_2/control_flow_collections/await_for_null_test.dart
index 47529b273d7..482cbffb903 100644
--- a/tests/language_2/control_flow_collections/await_for_null_test.dart
+++ b/tests/language_2/control_flow_collections/await_for_null_test.dart
@@ -11,11 +11,14 @@ void main() {
   asyncTest(() async {
     // Null stream.
     Stream<int> nullStream = null;
-    asyncExpectThrows<Error>(
-        () async => <int>[await for (var i in nullStream) 1]);
-    asyncExpectThrows<Error>(
-        () async => <int, int>{await for (var i in nullStream) 1: 1});
-    asyncExpectThrows<Error>(
-        () async => <int>{await for (var i in nullStream) 1});
+    asyncExpectThrows<Error>(() async {
+      <int>[await for (var i in nullStream) 1];
+    }());
+    asyncExpectThrows<Error>(() async {
+      <int, int>{await for (var i in nullStream) 1: 1};
+    }());
+    asyncExpectThrows<Error>(() async {
+      <int>{await for (var i in nullStream) 1};
+    }());
   });
 }
diff --git a/tests/language_2/control_flow_collections/await_for_test.dart b/tests/language_2/control_flow_collections/await_for_test.dart
index 402820be270..d6611433c7c 100644
--- a/tests/language_2/control_flow_collections/await_for_test.dart
+++ b/tests/language_2/control_flow_collections/await_for_test.dart
@@ -32,17 +32,28 @@ Future<void> testList() async {
   Expect.listEquals(list, <int>[await for (var i in stream(list)) i]);
 
   // Await for at beginning.
-  Expect.listEquals(list, <int>[await for (var i in stream([1, 2])) i, 3, 4]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([1, 2])) i,
+    3,
+    4
+  ]);
 
   // Await for in middle.
-  Expect.listEquals(list, <int>[1, await for (var i in stream([2, 3])) i, 4]);
+  Expect.listEquals(list, <int>[
+    1,
+    await for (var i in stream([2, 3])) i,
+    4
+  ]);
 
   // Await for at end.
-  Expect.listEquals(list, <int>[1, 2, await for (var i in stream([3, 4])) i]);
+  Expect.listEquals(list, <int>[
+    1,
+    2,
+    await for (var i in stream([3, 4])) i
+  ]);
 
   // Empty await for.
-  Expect.listEquals(list,
-      <int>[1, 2, await for (var i in stream([])) i, 3, 4]);
+  Expect.listEquals(list, <int>[1, 2, await for (var i in stream([])) i, 3, 4]);
 
   // Multiple await fors.
   Expect.listEquals(list, <int>[
@@ -52,38 +63,58 @@ Future<void> testList() async {
   ]);
 
   // Spread inside await for.
-  Expect.listEquals(list,
-      <int>[await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]
+  ]);
 
   // If inside await for.
-  Expect.listEquals(list,
-      <int>[await for (var i in stream([1, 9, 2, 3, 9, 4])) if (i != 9) i]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([1, 9, 2, 3, 9, 4]))
+      if (i != 9) i
+  ]);
 
   // Else inside await for.
-  Expect.listEquals(list,
-      <int>[await for (var i in stream([1, -2, 3, -4])) if (i < 0) -i else i]);
+  Expect.listEquals(list, <int>[
+    await for (var i in stream([1, -2, 3, -4]))
+      if (i < 0) -i else i
+  ]);
 
   // For inside await for.
   Expect.listEquals(list, <int>[
-    await for (var i in stream([0, 2])) for (var j = 1; j <= 2; j++) i + j
+    await for (var i in stream([0, 2]))
+      for (var j = 1; j <= 2; j++) i + j
   ]);
 
   // Does not flatten nested collection literal.
-  Expect.listEquals([1], [await for (var i in stream([1])) [i]].first);
-  Expect.mapEquals({1: 1}, [await for (var i in stream([1])) {i: i}].first);
-  Expect.setEquals({1}, [await for (var i in stream([1])) {i}].first);
+  Expect.listEquals(
+      [1],
+      [
+        await for (var i in stream([1])) [i]
+      ].first);
+  Expect.mapEquals(
+      {1: 1},
+      [
+        await for (var i in stream([1])) {i: i}
+      ].first);
+  Expect.setEquals(
+      {1},
+      [
+        await for (var i in stream([1])) {i}
+      ].first);
 
   // Downcast stream.
   Object obj = stream([1, 2, 3, 4]);
   Expect.listEquals(list, <int>[await for (var n in obj) n]);
 
   // Downcast variable.
-  Expect.listEquals(list,
-      <int>[await for (int n in numStream([1, 2, 3, 4])) n]);
+  Expect.listEquals(list, <int>[
+    await for (int n in numStream([1, 2, 3, 4])) n
+  ]);
 
   // Downcast element.
-  Expect.listEquals(list,
-      <int>[await for (num n in numStream([1, 2, 3, 4])) n]);
+  Expect.listEquals(list, <int>[
+    await for (num n in numStream([1, 2, 3, 4])) n
+  ]);
 }
 
 Future<void> testMap() async {
@@ -91,26 +122,30 @@ Future<void> testMap() async {
   Expect.mapEquals(map, <int, int>{await for (var i in stream(list)) i: i});
 
   // Await for at beginning.
-  Expect.mapEquals(map,
-      <int, int>{await for (var i in stream([1, 2])) i: i, 3: 3, 4: 4});
-
-  // Await for in middle.
-  Expect.mapEquals(map,
-      <int, int>{1: 1, await for (var i in stream([2, 3])) i: i, 4: 4});
-
-  // Await for at end.
-  Expect.mapEquals(map,
-      <int, int>{1: 1, 2: 2, await for (var i in stream([3, 4])) i: i});
-
-  // Empty await for.
   Expect.mapEquals(map, <int, int>{
-    1: 1,
-    await for (var i in stream([])) i: i,
-    2: 2,
+    await for (var i in stream([1, 2])) i: i,
     3: 3,
     4: 4
   });
 
+  // Await for in middle.
+  Expect.mapEquals(map, <int, int>{
+    1: 1,
+    await for (var i in stream([2, 3])) i: i,
+    4: 4
+  });
+
+  // Await for at end.
+  Expect.mapEquals(map, <int, int>{
+    1: 1,
+    2: 2,
+    await for (var i in stream([3, 4])) i: i
+  });
+
+  // Empty await for.
+  Expect.mapEquals(map,
+      <int, int>{1: 1, await for (var i in stream([])) i: i, 2: 2, 3: 3, 4: 4});
+
   // Multiple await fors.
   Expect.mapEquals(map, <int, int>{
     await for (var i in stream([1])) i: i,
@@ -120,18 +155,22 @@ Future<void> testMap() async {
 
   // Spread inside await for.
   Expect.mapEquals(map, <int, int>{
-    await for (var i in stream([0, 2]))
-      ...<int, int>{1 + i: 1 + i, 2 + i: 2 + i}
+    await for (var i in stream([0, 2])) ...<int, int>{
+      1 + i: 1 + i,
+      2 + i: 2 + i
+    }
   });
 
   // If inside await for.
   Expect.mapEquals(map, <int, int>{
-    await for (var i in stream([1, 9, 2, 3, 9, 4])) if (i != 9) i: i
+    await for (var i in stream([1, 9, 2, 3, 9, 4]))
+      if (i != 9) i: i
   });
 
   // Else inside await for.
   Expect.mapEquals(map, <int, int>{
-    await for (var i in stream([1, -2, 3, -4])) if (i < 0) -i: -i else i: i
+    await for (var i in stream([1, -2, 3, -4]))
+      if (i < 0) -i: -i else i: i
   });
 
   // For inside await for.
@@ -145,12 +184,14 @@ Future<void> testMap() async {
   Expect.mapEquals(map, <int, int>{await for (var n in obj) n: n});
 
   // Downcast variable.
-  Expect.mapEquals(map,
-      <int, int>{await for (int n in numStream([1, 2, 3, 4])) n: n});
+  Expect.mapEquals(map, <int, int>{
+    await for (int n in numStream([1, 2, 3, 4])) n: n
+  });
 
   // Downcast element.
-  Expect.mapEquals(map,
-      <int, int>{await for (num n in numStream([1, 2, 3, 4])) n: n});
+  Expect.mapEquals(map, <int, int>{
+    await for (num n in numStream([1, 2, 3, 4])) n: n
+  });
 }
 
 Future<void> testSet() async {
@@ -158,17 +199,28 @@ Future<void> testSet() async {
   Expect.setEquals(set, <int>{await for (var i in stream(list)) i});
 
   // Await for at beginning.
-  Expect.setEquals(set, <int>{await for (var i in stream([1, 2])) i, 3, 4});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([1, 2])) i,
+    3,
+    4
+  });
 
   // Await for in middle.
-  Expect.setEquals(set, <int>{1, await for (var i in stream([2, 3])) i, 4});
+  Expect.setEquals(set, <int>{
+    1,
+    await for (var i in stream([2, 3])) i,
+    4
+  });
 
   // Await for at end.
-  Expect.setEquals(set, <int>{1, 2, await for (var i in stream([3, 4])) i});
+  Expect.setEquals(set, <int>{
+    1,
+    2,
+    await for (var i in stream([3, 4])) i
+  });
 
   // Empty await for.
-  Expect.setEquals(set,
-      <int>{1, await for (var i in stream([])) i, 2, 3, 4});
+  Expect.setEquals(set, <int>{1, await for (var i in stream([])) i, 2, 3, 4});
 
   // Multiple await fors.
   Expect.setEquals(set, <int>{
@@ -178,36 +230,58 @@ Future<void> testSet() async {
   });
 
   // Spread inside await for.
-  Expect.setEquals(set,
-      <int>{await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([0, 2])) ...<int>[1 + i, 2 + i]
+  });
 
   // If inside await for.
-  Expect.setEquals(set,
-      <int>{await for (var i in stream([1, 9, 2, 3, 9, 4])) if (i != 9) i});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([1, 9, 2, 3, 9, 4]))
+      if (i != 9) i
+  });
 
   // Else inside await for.
-  Expect.setEquals(set,
-      <int>{await for (var i in stream([1, -2, 3, -4])) if (i < 0) -i else i});
+  Expect.setEquals(set, <int>{
+    await for (var i in stream([1, -2, 3, -4]))
+      if (i < 0) -i else i
+  });
 
   // For inside await for.
   Expect.setEquals(set, <int>{
-    await for (var i in stream([0, 2])) for (var j = 1; j <= 2; j++) i + j
+    await for (var i in stream([0, 2]))
+      for (var j = 1; j <= 2; j++) i + j
   });
 
   // Does not flatten nested collection literal.
-  Expect.listEquals([1], {await for (var i in stream([1])) [i]}.first);
-  Expect.mapEquals({1: 1}, {await for (var i in stream([1])) {i: i}}.first);
-  Expect.setEquals({1}, {await for (var i in stream([1])) {i}}.first);
+  Expect.listEquals(
+      [1],
+      {
+        await for (var i in stream([1])) [i]
+      }.first);
+  Expect.mapEquals(
+      {1: 1},
+      {
+        await for (var i in stream([1])) {i: i}
+      }.first);
+  Expect.setEquals(
+      {1},
+      {
+        await for (var i in stream([1])) {i}
+      }.first);
 
   // Downcast stream.
   Object obj = stream([1, 2, 3, 4]);
   Expect.setEquals(set, <int>{await for (var n in obj) n});
 
   // Downcast variable.
-  Expect.setEquals(set, <int>{await for (int n in numStream([1, 2, 3, 4])) n});
+  Expect.setEquals(set, <int>{
+    await for (int n in numStream([1, 2, 3, 4])) n
+  });
 
   // Downcast element.
-  Expect.setEquals(set, <int>{await for (num n in numStream([1, 2, 3, 4])) n});
+  Expect.setEquals(set, <int>{
+    await for (num n in numStream([1, 2, 3, 4])) n
+  });
 }
 
 Future<void> testDuplicateKeys() async {
@@ -243,28 +317,46 @@ Future<void> testKeyOrder() async {
   Expect.equals("1:a,2:a", map.keys.join(","));
   Expect.equals("2,4", map.values.join(","));
 
-  var set = <Equality>{e1a, await for (var i in stream([0, 1, 2])) keys[i]};
+  var set = <Equality>{
+    e1a,
+    await for (var i in stream([0, 1, 2])) keys[i]
+  };
   Expect.equals("1:a,2:a", set.join(","));
 }
 
 Future<void> testRuntimeErrors() async {
   // Cast variable.
   dynamic nonStream = 3;
-  asyncExpectThrows<TypeError>(
-      () async => <int>[await for (int i in nonStream) 1]);
-  asyncExpectThrows<TypeError>(
-      () async => <int, int>{await for (int i in nonStream) 1: 1});
-  asyncExpectThrows<TypeError>(
-      () async => <int>{await for (int i in nonStream) 1});
+  asyncExpectThrows<TypeError>(() async {
+    <int>[await for (int i in nonStream) 1];
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int, int>{await for (int i in nonStream) 1: 1};
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int>{await for (int i in nonStream) 1};
+  }());
 
   // Wrong element type.
   dynamic nonInt = "string";
-  asyncExpectThrows<TypeError>(
-      () async => <int>[await for (var i in stream([1])) nonInt]);
-  asyncExpectThrows<TypeError>(
-      () async => <int, int>{await for (var i in stream([1])) nonInt: 1});
-  asyncExpectThrows<TypeError>(
-      () async => <int, int>{await for (var i in stream([1])) 1: nonInt});
-  asyncExpectThrows<TypeError>(
-      () async => <int>{await for (var i in stream([1])) nonInt});
+  asyncExpectThrows<TypeError>(() async {
+    <int>[
+      await for (var i in stream([1])) nonInt
+    ];
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int, int>{
+      await for (var i in stream([1])) nonInt: 1
+    };
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int, int>{
+      await for (var i in stream([1])) 1: nonInt
+    };
+  }());
+  asyncExpectThrows<TypeError>(() async {
+    <int>{
+      await for (var i in stream([1])) nonInt
+    };
+  }());
 }
diff --git a/tests/lib/js/js_util/async_test.dart b/tests/lib/js/js_util/async_test.dart
index 9172fe781dd..7e3316bca5b 100644
--- a/tests/lib/js/js_util/async_test.dart
+++ b/tests/lib/js/js_util/async_test.dart
@@ -42,8 +42,9 @@ main() {
   }
 
   Future<void> testRejectedPromise() async {
-    asyncExpectThrows<String>(() => js_util.promiseToFuture(rejectedPromise),
-        (String error) => error == 'rejected');
+    final String error = await asyncExpectThrows<String>(
+        js_util.promiseToFuture(rejectedPromise));
+    expect(error, equals('rejected'));
   }
 
   Future<void> testReturnResolvedPromise() async {
diff --git a/tests/lib_2/js/js_util/async_test.dart b/tests/lib_2/js/js_util/async_test.dart
index 6a4935893aa..a9131575c55 100644
--- a/tests/lib_2/js/js_util/async_test.dart
+++ b/tests/lib_2/js/js_util/async_test.dart
@@ -44,8 +44,9 @@ main() {
   }
 
   Future<void> testRejectedPromise() async {
-    asyncExpectThrows<String>(() => js_util.promiseToFuture(rejectedPromise),
-        (String error) => error == 'rejected');
+    final String error = await asyncExpectThrows<String>(
+        js_util.promiseToFuture(rejectedPromise));
+    expect(error, equals('rejected'));
   }
 
   Future<void> testReturnResolvedPromise() async {
diff --git a/tests/standalone/io/http_parser_header_add_test.dart b/tests/standalone/io/http_parser_header_add_test.dart
index 4421733f4d9..4fdf602886d 100644
--- a/tests/standalone/io/http_parser_header_add_test.dart
+++ b/tests/standalone/io/http_parser_header_add_test.dart
@@ -17,19 +17,15 @@ Future<void> testFormatException() async {
     request.response.close();
   });
 
-  final completer = Completer<void>();
   // The ’ character is U+2019 RIGHT SINGLE QUOTATION MARK.
   final client = HttpClient()..userAgent = 'Bob’s browser';
-  asyncExpectThrows<FormatException>(() async {
-    try {
-      await client.open("CONNECT", "127.0.0.1", server.port, "/");
-    } finally {
-      client.close(force: true);
-      server.close();
-      completer.complete();
-    }
-  });
-  await completer.future;
+  try {
+    await asyncExpectThrows<FormatException>(
+        client.open("CONNECT", "127.0.0.1", server.port, "/"));
+  } finally {
+    client.close(force: true);
+    server.close();
+  }
 }
 
 main() {
diff --git a/tests/standalone/io/io_override_test.dart b/tests/standalone/io/io_override_test.dart
index fdd49ba88cf..4fe9c6662ff 100644
--- a/tests/standalone/io/io_override_test.dart
+++ b/tests/standalone/io/io_override_test.dart
@@ -161,17 +161,17 @@ class LinkMock extends FileSystemEntity implements Link {
 }
 
 Future<Socket> socketConnect(dynamic host, int port,
-    {dynamic sourceAddress, int sourcePort = 0, Duration? timeout}) {
+    {dynamic sourceAddress, int sourcePort = 0, Duration? timeout}) async {
   throw "";
 }
 
 Future<ConnectionTask<Socket>> socketStartConnect(dynamic host, int port,
-    {dynamic sourceAddress, int sourcePort = 0}) {
+    {dynamic sourceAddress, int sourcePort = 0}) async {
   throw "";
 }
 
 Future<ServerSocket> serverSocketBind(dynamic address, int port,
-    {int backlog: 0, bool v6Only: false, bool shared: false}) {
+    {int backlog: 0, bool v6Only: false, bool shared: false}) async {
   throw "";
 }
 
@@ -217,9 +217,9 @@ Future<Null> ioOverridesRunTest() async {
       Expect.identical(
           _mockFileSystemEvent, new Directory("directory").watch());
       Expect.isTrue(new Link("link") is LinkMock);
-      asyncExpectThrows(() async => await Socket.connect(null, 0));
-      asyncExpectThrows(() async => await Socket.startConnect(null, 0));
-      asyncExpectThrows(() async => await ServerSocket.bind(null, 0));
+      asyncExpectThrows(Socket.connect(null, 0));
+      asyncExpectThrows(Socket.startConnect(null, 0));
+      asyncExpectThrows(ServerSocket.bind(null, 0));
       Expect.isTrue(stdin is StdinMock);
       Expect.identical(stdout, stdoutMock);
       Expect.identical(stderr, stderrMock);
diff --git a/tests/standalone_2/io/http_parser_header_add_test.dart b/tests/standalone_2/io/http_parser_header_add_test.dart
index 7c86ff247d0..e597ea31272 100644
--- a/tests/standalone_2/io/http_parser_header_add_test.dart
+++ b/tests/standalone_2/io/http_parser_header_add_test.dart
@@ -19,19 +19,15 @@ Future<void> testFormatException() async {
     request.response.close();
   });
 
-  final completer = Completer<void>();
   // The ’ character is U+2019 RIGHT SINGLE QUOTATION MARK.
   final client = HttpClient()..userAgent = 'Bob’s browser';
-  asyncExpectThrows<FormatException>(() async {
-    try {
-      await client.open("CONNECT", "127.0.0.1", server.port, "/");
-    } finally {
-      client.close(force: true);
-      server.close();
-      completer.complete();
-    }
-  });
-  await completer.future;
+  try {
+    await asyncExpectThrows<FormatException>(
+        client.open("CONNECT", "127.0.0.1", server.port, "/"));
+  } finally {
+    client.close(force: true);
+    server.close();
+  }
 }
 
 main() {