[dart2wasm] Fix location RangeError when using part

Here's a minimal repro that this CL fixes:

`ui.dart`

```dart
library dart.ui;

import 'dart:ffi';

part 'foo.dart';
```

`foo.dart`

```dart
part of dart.ui;

@Native<Void Function()>(symbol: 'foo_func', isLeaf: true)
external void foo_func();
```

When compiling with `compile_platform.dart` with `--target=dart2wasm`, the following error appears:


```
Unhandled exception:
Verification error: Target=wasm, VerificationStage.afterModularTransformations: Invalid location with target 'wasm' on FunctionNode() (FunctionNode): RangeError (offset): Invalid value: Not in inclusive range 0..56: 91
Context: 'foo_func_$import'.
Node: 'FunctionNode()'.
#0      VerificationErrorListener.reportError (package:kernel/verifier.dart:81:5)
#1      VerifyingVisitor.problem (package:kernel/verifier.dart:222:14)
#2      VerifyingVisitor._getLocation (package:kernel/verifier.dart:1361:7)
#3      VerifyingVisitor._hasLocation (package:kernel/verifier.dart:1370:26)
#4      VerifyingVisitor.getSameLibraryLastSeenTreeNode (package:kernel/verifier.dart:1342:28)
#5      VerifyingVisitor.localContext (package:kernel/verifier.dart:1382:24)
#6      VerifyingVisitor.defaultDartType (package:kernel/verifier.dart:1491:41)
#7      Visitor.visitVoidType (package:kernel/visitor.dart:1309:37)
#8      VoidType.accept (package:kernel/ast.dart:11190:42)
#9      FunctionNode.visitChildren (package:kernel/ast.dart:3919:16)
#10     VerifyingVisitor.visitChildren (package:kernel/verifier.dart:259:10)
#11     VerifyingVisitor.visitWithLocalScope (package:kernel/verifier.dart:266:5)
#12     VerifyingVisitor.visitFunctionNode (package:kernel/verifier.dart:721:5)
#13     FunctionNode.accept (package:kernel/ast.dart:3908:38)
#14     VerifyingVisitor.visitProcedure (package:kernel/verifier.dart:620:19)
#15     Procedure.accept (package:kernel/ast.dart:3311:40)
#16     visitList (package:kernel/ast.dart:14488:14)
#17     Library.visitChildren (package:kernel/ast.dart:591:5)
#18     VerifyingVisitor.visitChildren (package:kernel/verifier.dart:259:10)
#19     VerifyingVisitor.defaultTreeNode (package:kernel/verifier.dart:196:5)
#20     TreeVisitor.visitLibrary (package:kernel/visitor.dart:503:35)
#21     VerifyingVisitor.visitLibrary (package:kernel/verifier.dart:367:11)
#22     Library.accept (package:kernel/ast.dart:577:38)
#23     visitList (package:kernel/ast.dart:14488:14)
#24     Component.visitChildren (package:kernel/ast.dart:14320:5)
#25     VerifyingVisitor.visitChildren (package:kernel/verifier.dart:259:10)
#26     VerifyingVisitor.visitComponent (package:kernel/verifier.dart:342:7)
#27     Component.accept (package:kernel/ast.dart:14313:38)
#28     VerifyingVisitor.check (package:kernel/verifier.dart:171:15)
#29     verifyComponent (package:kernel/verifier.dart:69:20)
...
```

The issue seems to be that after doing this native transformation, the node's `fileUri` references the enclosing library (`ui.dart` above), but the `node.location` references the actual source file (`foo.dart` above) indirectly through `node.fileOffset`.

This ends up being an issue when compiling the platform dill in Google3,   but I didn't look into why `flutter build web --wasm` isn't broken.

Internal bug: b/292172146

Change-Id: I2b8d7d215b2c36354860257ce651d50168e9523d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/315360
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Jia Hao Goh <jiahaog@google.com>

pkg/dart2wasm/lib/ffi_native_transformer.dart

@@ -191,7 +191,7 @@ class WasmFfiNativeTransformer extends FfiNativeTransformer {
         FunctionNode(null,
             positionalParameters: wasmImportProcedureArgs,
             returnType: retWasmType_),
-        fileUri: currentLibrary.fileUri,
+        fileUri: node.fileUri,
         isExternal: true,
         isStatic: true,
         isSynthetic: true)