Small PbList improvements (#906)

- Reduce binary size increase caused by cl/586928869 by adding
`dart2js:never-inline` pragmas to `add`, `addAll`, `clear`.

- Override `isEmpty`, `isNotEmpty` avoid virtual calls to `length`.

- Override `get iterator` to use the known iterator type of
`_wrappedList`, which is always a `_GrowableList`.

cl/587627017

protobuf/lib/src/protobuf/pb_list.dart

@@ -45,13 +45,15 @@ class PbList<E> extends ListBase<E> {
         _check = _checkNotNull;
 
   @override
+  @pragma('dart2js:never-inline')
   void add(E element) {
     _checkModifiable('add');
     _check(element);
     _wrappedList.add(element);
   }
 
   @override
+  @pragma('dart2js:never-inline')
   void addAll(Iterable<E> iterable) {
     _checkModifiable('addAll');
     iterable.forEach(_check);
@@ -74,6 +76,7 @@ class PbList<E> extends ListBase<E> {
   }
 
   @override
+  @pragma('dart2js:never-inline')
   void clear() {
     _checkModifiable('clear');
     _wrappedList.clear();
@@ -163,6 +166,16 @@ class PbList<E> extends ListBase<E> {
   @override
   int get length => _wrappedList.length;
 
+  @override
+  bool get isEmpty => _wrappedList.isEmpty;
+
+  @override
+  bool get isNotEmpty => _wrappedList.isNotEmpty;
+
+  @override
+  @pragma('dart2js:never-inline')
+  Iterator<E> get iterator => _wrappedList.iterator;
+
   @override
   set length(int newLength) {
     _checkModifiable('set length');