Update getList and getMap return types

This improves iteration performance by making `moveNext` and `current`
direct calls in kernel. google#902 does not have the same effect on `moveNext`
and `current` calls, this change is needed even with google#902.

This change was not possible before google#880 as users could override the
list and map types to types that are not `PbList`s or `PbMap`s.

protobuf/lib/src/protobuf/field_set.dart

@@ -370,7 +370,7 @@ class _FieldSet {
   }
 
   /// The implementation of a generated getter for repeated fields.
-  List<T> _$getList<T>(int index) {
+  PbList<T> _$getList<T>(int index) {
     final value = _values[index];
     if (value != null) return value;
 
@@ -387,9 +387,9 @@ class _FieldSet {
   }
 
   /// The implementation of a generated getter for map fields.
-  Map<K, V> _$getMap<K, V>(GeneratedMessage parentMessage, int index) {
+  PbMap<K, V> _$getMap<K, V>(GeneratedMessage parentMessage, int index) {
     final value = _values[index];
-    if (value != null) return value as Map<K, V>;
+    if (value != null) return value as PbMap<K, V>;
 
     final fi = _nonExtensionInfoByIndex(index) as MapFieldInfo<K, V>;
     assert(fi.isMapField);

protobuf/lib/src/protobuf/generated_message.dart

@@ -417,11 +417,12 @@ abstract class GeneratedMessage {
 
   /// For generated code only.
   /// @nodoc
-  List<T> $_getList<T>(int index) => _fieldSet._$getList<T>(index);
+  PbList<T> $_getList<T>(int index) => _fieldSet._$getList<T>(index);
 
   /// For generated code only.
   /// @nodoc
-  Map<K, V> $_getMap<K, V>(int index) => _fieldSet._$getMap<K, V>(this, index);
+  PbMap<K, V> $_getMap<K, V>(int index) =>
+      _fieldSet._$getMap<K, V>(this, index);
 
   /// For generated code only.
   /// @nodoc