docs(): add gotcha for dart not having generic methods.

Author: jelbourn

docs/dart/writing-compatible-typescript.md

@@ -41,4 +41,5 @@ Here's a list of gotchas to keep in mind when writing TypeScript code that will
 * **Default values need to be constants.** This mean that code like `function myFunction(arg1: string = callToSomething()) {}` will not compile.
 * **The const keyword must have a const value.** Because of that, we cannot do `const x = a + b;` even if the value of `x`, `a` and `b` will not change.
 * **Lambdas need to abide to the type required.** Meaning that if a function requires a function that takes one argument, the lambda cannot be `() => {}`. Use `_` for temporary parameters. This is notable in Promises.
-* **Dynamic return values fat arrows can't return non-primitive types without casting.** For example, a fat arrow that return a promise (or Future) generates a warning if using directly, like so: `promiseA.then(() => promiseB)`. In this case simply using a block works as expected; `promiseA.then(() => { return promiseB; })`.
+* **Dynamic return values fat arrows can't return non-primitive types without casting.** For example, a fat arrow that return a promise (or Future) generates a warning if using directly, like so: `promiseA.then(() => promiseB)`. In this case simply using a block works as expected; `promiseA.then(() => { return promiseB; })`.
+* **Dart does not have [generic methods](http://www.typescriptlang.org/Handbook#generics)** ([issue](https://github.com/dart-lang/sdk/issues/254)).