…-candidate.0

…nditional imports (flutter#53307)

Users of packages that have specialized code for the VM (which supports
FFI) use conditional imports based on `dart.library.ffi`. We don't want
the VM-specific code to be used for web in dart2wasm.

As a result we're going to make `dart.library.ffi` be false in
conditional imports (as well as in
`const bool.fromEnvironment('dart.library.ffi')`).

Issue dart-lang/sdk#55948
Issue flutter/flutter#149984

…er#53304)

The issue in [0] was fixed. So we no longer have to use conditional
imports and dart2wasm vs dart2js specific implementations.

Instead we use the new extension methods introduced due to [0], namely
`Object.toExternalReference` and `ExternalDartReference.toDartObject`.

[0] dart-lang/sdk#55183