[tests] update await_for_test to test behavior variations.

This introduces an asyncExpectThrowsWhen to async_helper, similar to
Expect.throwsWhen, to allow specifying semantics under behavior
variations in this test.

We also update await_for_test to no longer skip expectations in dart2js.

Change-Id: Ie147f74f384a0e196e40b75c59fe585f011ede49
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/351721
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>

pkg/async_helper/lib/async_helper.dart

@@ -142,3 +142,16 @@ Future<T> asyncExpectThrows<T extends Object>(Future<void> result,
     return error;
   });
 }
+
+/// Checks that the asynchronous [result] throws a [T] if and only if
+/// [condition] is `true`.
+///
+/// When [condition] is `false`, [result] is expected to complete without
+/// errors.
+Future<T?> asyncExpectThrowsWhen<T extends Object>(
+    bool condition, Future<void> result,
+    [String reason = ""]) {
+  return condition
+      ? asyncExpectThrows<T>(result, reason)
+      : result.then<T?>((_) => null);
+}

tests/language/control_flow_collections/await_for_test.dart

@@ -281,42 +281,46 @@ Future<void> testKeyOrder() async {
   Expect.equals("1:a,2:a", set.join(","));
 }
 
-Future<void> testRuntimeErrors() async {
-  // TODO(54798): split these and move them closer to each expectation.
-  if (!v.checkedParameters || !v.checkedImplicitDowncasts) {
-    return;
+asyncExpectThrowsTypeErrorOrNSM(Future<void> result) {
+  if (v.checkedParameters) {
+    return asyncExpectThrows<TypeError>(result);
+  } else {
+    return asyncExpectThrows<NoSuchMethodError>(result);
   }
+}
 
+Future<void> testRuntimeErrors() async {
   // Cast variable.
   dynamic nonStream = 3;
-  asyncExpectThrows<TypeError>(() async {
+  asyncExpectThrowsTypeErrorOrNSM(() async {
     <int>[await for (int i in nonStream) 1];
   }());
-  asyncExpectThrows<TypeError>(() async {
+  asyncExpectThrowsTypeErrorOrNSM(() async {
     <int, int>{await for (int i in nonStream) 1: 1};
   }());
-  asyncExpectThrows<TypeError>(() async {
+  asyncExpectThrowsTypeErrorOrNSM(() async {
     <int>{await for (int i in nonStream) 1};
   }());
 
   // Wrong element type.
   dynamic nonInt = "string";
-  asyncExpectThrows<TypeError>(() async {
+  asyncExpectThrowsWhen(v.checkedImplicitDowncasts, () async {
     <int>[
       await for (var i in stream([1])) nonInt
     ];
   }());
-  asyncExpectThrows<TypeError>(() async {
+  asyncExpectThrowsWhen(v.checkedImplicitDowncasts, () async {
     <int, int>{
       await for (var i in stream([1])) nonInt: 1
     };
   }());
-  asyncExpectThrows<TypeError>(() async {
+  asyncExpectThrowsWhen(v.checkedImplicitDowncasts, () async {
     <int, int>{
       await for (var i in stream([1])) 1: nonInt
     };
   }());
-  asyncExpectThrows<TypeError>(() async {
+
+  asyncExpectThrowsWhen(v.checkedImplicitDowncasts, () async {
     <int>{
       await for (var i in stream([1])) nonInt
     };