Platform (iOS, Android, etc.) should be a useable condition for conditional imports

I have multiple platform-specific implementations of an HTTP client and I would like to select the appropriate one for the platform e.g. to use one based on NSURLSession on iOS, cronet on Android, etc.

In C++, I would use preprocessor macros to solve this issue e.g.

```C++
HttpClient *client;
#ifdef IOS
#include
client = new NSURLAdapter();
#elif ...
```

In Python, I would do the configuration at runtime e.g.

```python
import platform

client: HttpClient
if platform.system() == "Windows":
import whatever
client = whatever.WindowsClient()
elif ...
```

In Dart, I don't know how to solve this problem except in the narrow case of differentiating between web and non-web (using conditional imports).

No files, tests, or entry points are named. Start by reviewing the conditional-import behavior described in this issue and the platform-specific examples, then define the language behavior and acceptance criteria for selecting implementations by platform.

Written by the indexing model from the issue text.