[ddc] fix windows tests for dynamic modules.

Using toFilePath produces an absolute path with `c:` some of our snapshots (like kenrel_worker) don't know how to parse.

Using the URI string representation includes the scheme and addresses the issues (eg. example failure from before  https://dart-ci.appspot.com/log/ddc-win-chrome/ddc-win-chrome/6835/dynamic_modules_suite/shared_const)

Change-Id: Id71faf7745e5843cff6fa06fd198576b7560acf3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/385301
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>

pkg/dynamic_modules/test/runner/ddc.dart

@@ -68,12 +68,12 @@ class DdcExecutor implements TargetExecutor {
       '--no-summarize',
       '--no-source-map',
       '--multi-root',
-      sourceDir.resolve('../../').toFilePath(),
+      '${sourceDir.resolve('../../')}',
       '--multi-root-scheme',
       rootScheme,
       '$rootScheme:/data/$testName/$source',
       '--dart-sdk-summary',
-      ddcSdkOutline.toFilePath(),
+      '$ddcSdkOutline',
       // Note: this needs to change if we ever intend to support packages within
       // the dynamic loading tests themselves
       '--packages=${repoRoot.toFilePath()}/.dart_tool/package_config.json',
@@ -100,12 +100,12 @@ class DdcExecutor implements TargetExecutor {
       '--target',
       'ddc',
       '--multi-root',
-      sourceDir.resolve('../../').toFilePath(),
+      '${sourceDir.resolve('../../')}',
       '--multi-root-scheme',
       rootScheme,
       '--packages-file=${repoRoot.toFilePath()}/.dart_tool/package_config.json',
       '--dart-sdk-summary',
-      ddcSdkOutline.toFilePath(),
+      '$ddcSdkOutline',
       '--source',
       '$rootScheme:/data/$testName/$source',
       '--output',