Allow application of external mixins to generated dart protos.

lib/testing/mixins.dart

Index: lib/testing/mixins.dart
diff --git a/lib/testing/mixins.dart b/lib/testing/mixins.dart
new file mode 100644
index 0000000000000000000000000000000000000000..694840c1e9b4bc6e84ffe3322b2c6a739862f2ed
--- /dev/null
+++ b/lib/testing/mixins.dart
@@ -0,0 +1,15 @@
+abstract class Mixin1 {
+  String get overriddenString => "mixin1";
+
+  String get interfaceString;
+  set interfaceString(String string);
+  bool hasInterfaceString();
+}
+
+abstract class Mixin2 {
+  String get overriddenString => "mixin2";
+
+  bool hasOverriddenHasMethod() => false;
+}
+
+abstract class Mixin3 {}