Add section on known bugs

For now stage this in the package:js README - that might not be the best
place for it long term since the details may change...

- Add a note that makes it explicit we expect all interop to use
  `package:js` instead of `dart:js`.
- Add notes about known limitations and differences between dart2js and
  DDC.

Change-Id: Ib4c967ea1435dd85f41f56646140352b125cee4c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107560
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

Author: natebosch

pkg/js/README.md

@@ -7,6 +7,9 @@ end-to-end example.
 
 ### Usage
 
+All Dart code interacting with JavaScript should use the utilities provided with
+`package:js`. Developers should avoid importing `dart:js` directly.
+
 #### Calling methods
 
 ```dart
@@ -94,7 +97,114 @@ DDC there will be no errors despite missing `allowInterop` calls, because DDC
 uses JS calling semantics by default. When compiling with Dart2JS the
 `allowInterop` utility must be used.
 
+#### Making a Dart function callable from JavaScript
+
+To provide a Dart function callable from JavaScript by name use a setter
+annotated with `@JS()`.
+
+```dart
+@JS()
+library callable_function;
+
+import 'package:js/js.dart';
+
+/// Allows assigning a function to be callable from `window.functionName()`
+@JS('functionName')
+external set _functionName(void Function() f);
+
+/// Allows calling the assigned function from Dart as well.
+@JS()
+external void functionName();
+
+void _someDartFunction() {
+  print('Hello from Dart!');
+}
+
+void main() {
+  _functionName = allowInterop(_someDartFunction);
+  // JavaScript code may now call `functionName()` or `window.functionName()`.
+}
+```
+
+## Known limitations and bugs
+
+### Differences betwenn Dart2JS and DDC
+
+Dart's production and development JavaScript compilers use different calling
+conventions and type representation, and therefore have different challenges in
+JavaScript interop. There are currently some know differences in behavior and
+bugs in one or both compilers.
+
+#### allowInterop is required in Dart2JS, optional in DDC
+
+DDC uses the same calling conventions as JavaScript and so Dart functions passed
+as callbacks can be invoked without modification. In Dart2JS the calling
+conventions are different and so `allowInterop` or `allowInteropCaptureThis`
+must be used for any callback.
+
+**Workaround:**: Always use `allowInterop` even when not required in DDC.
+
+#### Callbacks allow extra ignored arguments in DDC
+
+In JavaScript a caller may pass any number of "extra" arguments to a function
+and they will be ignored. DDC follows this behavior, Dart2JS will have a runtime
+error if a function is invoked with more arguments than expected.
+
+**Workaround:** Write functions that take the same number of arguments as will
+be passed from JavaScript. If the number is variable use optional positional
+arguments.
+
+#### DDC and Dart2JS have different representation for Maps
+
+Passing a `Map<String, String>` as an argument to a JavaScript function will
+have different behavior depending on the compiler. Calling something like
+`JSON.stringify()` will give different results.
+
+**Workaround:** Only pass object literals instead of Maps as arguments. For json
+specifically use `jsonEncode` in Dart rather than a JS alternative.
+
+#### Missing validation for anonymous factory constructors in DDC
+
+When using an `@anonymous` class to create JavaScript object literals Dart2JS
+will enforce that only named arguments are used, while DDC will allow positional
+arguments but may generate incorrect code.
+
+**Workaround:** Try builds in both development and release mode to get the full
+scope of static validation.
+
+### Sharp Edges
+
+Dart and JavaScript have different semantics and common patterns which makes it
+easy to make some mistakes, and difficult for the tools to provide safety. These
+sharp edges are known pitfalls.
+
+#### Lack of runtime type checking
+
+The return types of methods annotated with `@JS()` are not validated at runtime,
+so an incorrect type may "leak" into other Dart code and violate type system
+guarantees.
+
+**Workaround:** For any calls into JavaScript code that are not known to be safe
+in their return values, validate the results manually with `is` checks.
+
+#### List instances coming from JavaScript will always be `List<dynamic>`
+
+A JavaScript array does not have a reified element type, so an array returned
+from a JavaScript function cannot make guarantees about it's elements without
+inspecting each one. At runtime a check like `result is List` may succeed, while
+`result is List<String>` will always fail.
+
+**Workaround:** Use a `.cast<String>().toList()` call to get a `List` with the
+expected reified type at runtime.
+
+#### The `JsObject` type from `dart:js` can't be used with `@JS()` annotation
+
+`JsObject` and related code in `dart:js` uses a different approach and may not
+be passed as an argument to a method annotated with `@JS()`.
 
+**Workaround:** Avoid importing `dart:js` and only use the `package:js` provided
+approach. To handle object literals use `@anonymous` on an `@JS()` annotated
+class.
 
 ## Reporting issues