Added first draft of implementation-plan.md for generalized type aliases

accepted/future-releases/generalized-typedef-2018/implementation-plan.md

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+# Implementation Plan for the Q1 2019 Generalized Type Alias Feature
+
+Relevant documents:
+ - [Tracking issue](https://github.com/dart-lang/language/issues/115)
+ - [Feature specification](https://github.com/dart-lang/language/blob/master/accepted/future-releases/generalized-typedef-2018/feature-specification.md)
+
+## Implementation and Release plan
+
+This feature is non-breaking, because it is concerned with the introduction of
+support for new syntactic forms.
+Still, we will introduce it using an
+[experiments flag](https://github.com/dart-lang/sdk/blob/master/docs/process/experimental-flags.md)
+in order to enable a controlled deployment.
+
+
+### Phase 0 (Preliminaries)
+
+#### Release Flag
+
+The flag
+`--enable-experiment=nonfunction-type-aliases`
+must be passed for the changes to be enabled.
+In this phase, support for that flag is added to all tools.
+
+#### Tests
+
+The language team adds a set of tests for the new feature, in terms of
+declarations and usages in the following situations:
+
+- A type alias with and without type arguments is used
+  - as a type annotation for a variable of various kinds.
+  - as a type argument of a class or another type alias.
+  - as part of a function type.
+  - in a function declaration as return type or parameter type.
+  - as an expression.
+  - as the type in an `on` clause.
+  - in a type test (`is`).
+  - in a type cast (`as`).
+- A type alias whose body is a class with or without type arguments, is used in
+  - the `extends` clause of a class.
+  - the `with` clause of a class.
+  - the `implements` clause of a class or mixin.
+  - the `on` clause of a mixin.
+  - instance creation expressions, constant and non-constant.
+  - an invocation of a static method, getter, setter, and a tear-off of a
+    static method; note that the provision of type arguments
+    (`F<int>.m()`) is an error here.
+
+The co19 team start creating tests early, such that those tests can be
+used during implementation as well.
+
+### Phase 1 (Implementation)
+
+All tools implement syntactic support for type aliases of the form
+
+```dart
+typedef F<TypeArguments> = type;
+```
+
+where `type` can be any type, rather than just a function type.
+
+All tools implement support for using such type aliases, in all situations
+mentioned under phase 0.
+
+### Phase 2 (Release)
+
+The feature is released as part of the next stable Dart release.
+
+## Timeline
+
+Completion goals for the phases:
+
+- Phase 0: (TODO)
+- Phase 1: (TODO)
+- Phase 2: (TODO)