Set correct fileUri on FfiNative fields.
Set correct fileUri on FfiNative fields.

`currentLibrary.fileUri` points to the library file (e.g. ui.dart)
and not the class-containing `part` file (e.g. compositing.dart).
This causes `fileOffset` out-of-bounds and `location` to fail.

Failing stack trace:
Unhandled exception: Crash when compiling null, at character offset null:
RangeError (offset): Invalid value: Not in inclusive range 0..1257: 1543
#0      RangeError.checkValueInInterval (dart:core/errors.dart:282:7)
#1      Source.getLocation (package:kernel/ast.dart:13919:16)
#2      Component.getLocation (package:kernel/ast.dart:13692:31)
#3      _getLocationInComponent (package:kernel/ast.dart:14212:22)
#4      Field._getLocationInEnclosingFile (package:kernel/ast.dart:2363:12)
#5      TreeNode._getLocationInEnclosingFile (package:kernel/ast.dart:206:20)
#6      TreeNode.location (package:kernel/ast.dart:202:12)
#7      _FfiUseSiteTransformer._ensureNativeTypeToDartType (package:vm/transformations/ffi_use_sites.dart:744:35)
[...]

TEST=Existing.
Change-Id: I17b3ab23abcb04aca046509e7cdbb177ed156e54
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/217186
Commit-Queue: Clement Skau <495593248b6158163719b90dde8357b589c8e5d0@google.com>
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk