Add generic types for strong mode

lib/src/protobuf/field_info.dart

Index: lib/src/protobuf/field_info.dart
diff --git a/lib/src/protobuf/field_info.dart b/lib/src/protobuf/field_info.dart
index d9235756a452b23b7ff3884dfdaa43a5c4a97ff7..0a7886963ff174e95462c06b98a620cd9d093963 100644
--- a/lib/src/protobuf/field_info.dart
+++ b/lib/src/protobuf/field_info.dart
@@ -7,7 +7,7 @@ part of protobuf;
 /**
  * An object representing a protobuf message field.
  */
-class FieldInfo {
+class FieldInfo<T> {
   final String name;
   final int tagNumber;
   final int index; // index of the field's value. Null for extensions.
@@ -46,7 +46,7 @@ class FieldInfo {
       [this.valueOf])
       : this.type = type,
         this.check = check,
-        this.makeDefault = (() => new PbList(check: check)) {
+        this.makeDefault = (() => new PbList<T>(check: check)) {

[skybrian, 2016/04/01 18:01:40]

This is a bit inconsistent.

For a non-repeated field, T is the field's type.
For a repeated field, T is the *base type* and List<T> is the field type. (But
it's really a PbList or perhaps some other kind of list if a mixin overrides
it.)

Similarly for extensions. As a result, we can't really write generic methods
that work uniformly with non-repeated and repeated fields. (There are internal
methods that work that way, but they are currently untyped.)

I think I'm just going to live with it for now.

     assert(name != null);
     assert(tagNumber != null);
     assert(_isRepeated(type));
@@ -121,5 +121,14 @@ class FieldInfo {
     }
   }
 
+  /// Creates a repeated field to be attached to the given message.
+  ///
+  /// Delegates actual list creation to the message, so that it can
+  /// be overridden by a mixin.
+  List<T> _createRepeatedField(GeneratedMessage m) {
+    assert(isRepeated);
+    return m.createRepeatedField/*<T>*/(tagNumber, this);
+  }
+
   String toString() => name;
 }