Reland "[test] Add paths to chrome and firefox to the environment when present"

This is a reland of 40c79c8

The build recipes now use forward slashes on Windows to pass the path to
the browser executable to test.py.

Original change's description:
> [test] Add paths to chrome and firefox to the environment when present
>
> This adds a CHROME_PATH or FIREFOX_PATH environment variable to the
> test's evnironment which contains a path to the browser's executable.
> This is only set if the --chrome or --firefox argument was passed to
> test.py.
>
> #42417
>
> Change-Id: I711de6ab91b8ffbbe72c7fb1530b68c4189a4b8a
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153209
> Commit-Queue: Alexander Thomas <[email protected]>
> Reviewed-by: William Hesse <[email protected]>

Change-Id: Ib61b8c6424fa7bce525d76d8b94756a215d28c1e
Cq-Include-Trybots: dart/try:dart2js-strong-win-x64-chrome-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153980
Commit-Queue: Alexander Thomas <[email protected]>
Commit-Queue: William Hesse <[email protected]>
Auto-Submit: Alexander Thomas <[email protected]>
Reviewed-by: William Hesse <[email protected]>

Author: athomas

pkg/test_runner/lib/src/test_suite.dart

@@ -55,14 +55,16 @@ abstract class TestSuite {
   TestSuite(this.configuration, this.suiteName, this.statusFilePaths) {
     _environmentOverrides = {
       'DART_CONFIGURATION': configuration.configurationDirectory,
+      if (Platform.isWindows) 'DART_SUPPRESS_WER': '1',
+      if (Platform.isWindows && configuration.copyCoreDumps)
+        'DART_CRASHPAD_HANDLER':
+            Uri.base.resolve(buildDir + '/crashpad_handler.exe').toFilePath(),
+      if (configuration.chromePath != null)
+        'CHROME_PATH': Uri.base.resolve(configuration.chromePath).toFilePath(),
+      if (configuration.firefoxPath != null)
+        'FIREFOX_PATH':
+            Uri.base.resolve(configuration.firefoxPath).toFilePath(),
     };
-    if (Platform.isWindows) {
-      _environmentOverrides['DART_SUPPRESS_WER'] = '1';
-      if (configuration.copyCoreDumps) {
-        _environmentOverrides['DART_CRASHPAD_HANDLER'] =
-            Path(buildDir + '/crashpad_handler.exe').absolute.toNativePath();
-      }
-    }
   }
 
   Map<String, String> get environmentOverrides => _environmentOverrides;