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..2fa2fdc698cc7eb0619562cccab022d34c2f83f8
--- /dev/null
+++ b/lib/testing/mixins.dart
@@ -0,0 +1,13 @@
+abstract class Mixin1 {

[skybrian, 2016/06/22 20:29:52]

Can we put this file under "test" instead of in "lib/testing"?

I think we can get away with a relative import in the .proto file:
"../../test/mixins.dart"

[frederikmutzel, 2016/06/23 12:30:16]

I'd really prefer to use a package import here. Reasons: 
- It makes it easier to move the generated dart file
- Clients are more likely to use package imports (and they probably should), so
it would be a better example
- Internally, it seems like people prefer putting only tests in test/ and
testing support libraries in testing/ (although protobuf doesn't seem to do
that)

I'm generally not a fan of relative imports, tooling seems to handle package
imports much better as well when refactoring.

+  String get overriddenString => "mixin1";
+
+  String get interfaceString;
+  set interfaceString(String string);
+  bool hasInterfaceString();
+}
+
+abstract class Mixin2 {
+  String get overriddenString => "mixin2";
+
+  bool hasOverriddenHasMethod() => false;
+}