Support `void` as generic argument.

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index 751ffe73f3af36d6ee4fcae4281157b17cee748e..a49c24531bb4648899eed6a0e7cb0bb8693def33 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -92,6 +92,7 @@ enum MessageKind {
   ASYNC_MODIFIER_ON_ABSTRACT_METHOD,
   ASYNC_MODIFIER_ON_CONSTRUCTOR,
   ASYNC_MODIFIER_ON_SETTER,
+  AWAIT_FORIN_VOID_GENERIC,
   AWAIT_MEMBER_NOT_FOUND,
   AWAIT_MEMBER_NOT_FOUND_IN_CLOSURE,
   BAD_INPUT_CHARACTER,
@@ -201,6 +202,7 @@ enum MessageKind {
   FINAL_FUNCTION_TYPE_PARAMETER,
   FINAL_WITHOUT_INITIALIZER,
   FORIN_NOT_ASSIGNABLE,
+  FORIN_VOID_GENERIC,
   FORMAL_DECLARED_CONST,
   FORMAL_DECLARED_STATIC,
   FUNCTION_TYPE_FORMAL_WITH_DEFAULT,
@@ -521,6 +523,19 @@ class MessageTemplate {
       MessageKind.VOID_VARIABLE: const MessageTemplate(
           MessageKind.VOID_VARIABLE, "Variable cannot be of type void."),
 
+      MessageKind.FORIN_VOID_GENERIC: const MessageTemplate(
+          MessageKind.FORIN_VOID_GENERIC,
+          "Generic type of Iterable cannot be of type void.",
+          howToFix: "Try casting the iterable.",

[Siggi Cherem (dart-lang), 2017/03/13 22:50:58]

can a cast succeed? Doesn't this error imply that there is no way to read the
elements of the collection?

[floitsch, 2017/03/14 13:30:49]

Removed the "howToFix". There doesn't seem to be a good suggestion.

+          examples: const [
+            """
+main() {
+  for (var x in new List<void>()) {
+  }
+}
+"""
+            ]),
+
       MessageKind.RETURN_VALUE_IN_VOID: const MessageTemplate(
           MessageKind.RETURN_VALUE_IN_VOID,
           "Cannot return value from void function."),
@@ -540,6 +555,20 @@ class MessageTemplate {
           MessageKind.NAMED_ARGUMENT_NOT_FOUND,
           "No named argument '#{argumentName}' found on method."),
 
+      MessageKind.AWAIT_FORIN_VOID_GENERIC: const MessageTemplate(
+          MessageKind.AWAIT_FORIN_VOID_GENERIC,
+          "Generic type of Stream cannot be of type void.",
+          howToFix: "Try casting the stream.",

[Siggi Cherem (dart-lang), 2017/03/13 22:50:58]

same

[floitsch, 2017/03/14 13:30:49]

ditto.

+          examples: const [
+            """
+import 'dart:async';
+main() async {
+  await for (var x in new StreamView<void>(null)) {
+  }
+}
+"""
+          ]),
+
       MessageKind.AWAIT_MEMBER_NOT_FOUND: const MessageTemplate(
           MessageKind.AWAIT_MEMBER_NOT_FOUND,
           "No member named 'await' in class '#{className}'.",