[dart2wasm] Fix Uri.base in d8

Currently when the .wasm file path passed to run_wasm.js is a relative
path, `Uri.base` becomes something like `file://test.wasm`, which is not
a valid file URI, so it causes crashes in `Uri.toFilePath`.

When the file path is relative add a omit `file://` prefix.

`Uri.base` values before and after:

- Before, relative .wasm path: `file://test.wasm` (invalid)
- Before, absolute .wasm path: `file:///home/user/test.wasm`

- After, relative .wasm path: `test.wasm` (fixed)
- After, absolute .wasm path: `file:///home/user/test.wasm` (same as before)

Change-Id: I0d1c43716e07a9ee926e7feeeab514c2c66bac16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/385700
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

pkg/dart2wasm/bin/run_wasm.js

@@ -364,7 +364,11 @@ if (argsSplit != -1) {
   self.Response = function () { }
 
   self.location = {}
-  self.location.href = 'file://' + args[wasmArg];
+  if (args[wasmArg].startsWith('/')) {
+    self.location.href = 'file://' + args[wasmArg];
+  } else {
+    self.location.href = args[wasmArg];
+  }
 
   // Signals `Stopwatch._initTicker` to use `Date.now` to get ticks instead of
   // `performance.now`, as it's not available in d8.

tests/web/wasm/uri_base_test.dart

@@ -0,0 +1,7 @@
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+void main() {
+  print(Uri.base.toFilePath()); // should not crash
+}

tests/web/web.status

@@ -57,6 +57,7 @@ code_motion_exception_test: Skip # Required V8 specific format of JavaScript err
 [ $compiler == dart2wasm && $runtime != d8 ]
 wasm/source_map_simple_test: SkipByDesign # Reads source map file using d8's readbuffer
 wasm/source_map_simple_optimized_test: SkipByDesign # Reads source map file using d8's readbuffer
+wasm/uri_base_test: SkipByDesign # Converts Uri.base to file path
 
 [ $compiler == dartk && $runtime == vm ]
 new_from_env_test: SkipByDesign # dart2js only test