Generalize argument type of PbList.from from List<T> to Iterable<T> (#1054)

This is more consistent with `List.from` and also more efficient as we can
avoid redundant `List` allocations when we have an `Iterable` instead of a
`List`. This commonly happens when we create a `PbList` from a chain of `map`,
`where` etc. calls.

Also update the the factory body to use `List.of` instead of `List.from` as we
know the `Iterable`'s element type. (`List.from` takes `Iterable<dynamic>`,
`List.of` takes `Iterable<T>`)

cl/810421932

Author: osa1

protobuf/CHANGELOG.md

@@ -4,10 +4,13 @@
 * Fix unknown enum handling in `GeneratedMessage.mergeFromProto3Json` when
   the `ignoreUnknownFields` optional argument is `true`. ([#853])
 * Add `BuilderInfo` methods to support protoc-plugin 23.0.0. ([#1047])
+* Generalize argument type of `PbList.from` from `List<T>` to `Iterable<T>`.
+  ([#1054])
 
 [#742]: https://github.com/google/protobuf.dart/pull/742
 [#853]: https://github.com/google/protobuf.dart/pull/853
 [#1047]: https://github.com/google/protobuf.dart/pull/1047
+[#1054]: https://github.com/google/protobuf.dart/pull/1054
 
 ## 4.2.0
 

protobuf/lib/src/protobuf/pb_list.dart

@@ -40,8 +40,8 @@ class PbList<E> extends ListBase<E> {
       _check = _checkNotNull,
       _isReadOnly = true;
 
-  PbList.from(List<E> from)
-    : _wrappedList = List<E>.from(from),
+  PbList.from(Iterable<E> from)
+    : _wrappedList = List<E>.of(from),
       _check = _checkNotNull;
 
   @override