Tweak dart2js source map rewriting (#2128)

- Remove a TODO referencing a closed issue. The issue was deduped to
  another which was closed without changes. Manual testing in a browser
  shows that the rewriting is still necessary.
- Extract a variable for the source map path as a Uri instead of
  computing it within the loop.
- Directly return URIs which are already `file://`. This would be a
  slight behavior change for the `runInBrowser.dart` synthetic source if
  it was ever put in the source map file with an absolute `file://`
  URI, but with the current (and apparently planned) dart2js output they
  do not have the file scheme. This is not a behavior change for any
  other cases where the source is a `file` URI, since it is no necessary
  to call `resolveUri` with an absolute path, and all `file` URI are
  absolute.
- Refactor for readability - use a list literal, `Map.update`, and a `switch`.

A sample run with a `Stopwatch` shows this reduces the non-IO overhead
by about 75% (out of 165ms in my sample with multiple suites).

pkgs/test_core/lib/src/runner/dart2js_compiler_pool.dart

diff --git a/pkgs/test_core/lib/src/runner/dart2js_compiler_pool.dart b/pkgs/test_core/lib/src/runner/dart2js_compiler_pool.dart
index 343ce0c..a13a65a 100644
--- a/pkgs/test_core/lib/src/runner/dart2js_compiler_pool.dart
+++ b/pkgs/test_core/lib/src/runner/dart2js_compiler_pool.dart
@@ -93,19 +93,24 @@
     });
   }
 
-  // TODO(nweiz): Remove this when sdk#17544 is fixed.
   /// Fix up the source map at [mapPath] so that it points to absolute file:
   /// URIs that are resolvable by the browser.
   void _fixSourceMap(String mapPath) {
-    var map = jsonDecode(File(mapPath).readAsStringSync());
+    var map = jsonDecode(File(mapPath).readAsStringSync()) as Map;
     var root = map['sourceRoot'] as String;
 
-    map['sources'] = map['sources'].map((Object? source) {
-      var url = Uri.parse('$root$source');
-      if (url.scheme != '' && url.scheme != 'file') return source;
-      if (url.path.endsWith('/runInBrowser.dart')) return '';
-      return p.toUri(mapPath).resolveUri(url).toString();
-    }).toList();
+    final mapUri = p.toUri(mapPath);
+    map.cast<String, List<Object?>>().update(
+          'sources',
+          (sources) => [
+            for (var source in sources)
+              switch (Uri.parse('$root$source')) {
+                Uri(hasScheme: true) && final uri => uri.toString(),
+                Uri(:final path) when path.endsWith('/runInBrowser.dart') => '',
+                final uri => mapUri.resolveUri(uri).toString(),
+              }
+          ],
+        );
 
     File(mapPath).writeAsStringSync(jsonEncode(map));
   }