[ddc] Update content-length to not be negative

The null safety migration changed the 'content-length' from null to
-1 but a separate change at the same time disallowed negative
values.

Now returning what the 'content-length' would be if the request was
a GET.

Fixes: https://github.com/dart-lang/sdk/issues/49483

Change-Id: I2ba3ce1fb604c0fac50f354f56b3d29de6465caa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/252005
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

pkg/dev_compiler/test/expression_compiler/expression_compiler_worker_shared.dart

@@ -936,22 +936,15 @@ class TestAssetServer {
     try {
       var entity = fileSystem.entityForUri(uri);
       if (await entity.existsAsyncIfPossible()) {
-        if (request.method == 'HEAD') {
-          var headers = {
-            'content-length': '-1',
-            ...request.headers,
-          };
-          return Response.ok(null, headers: headers);
-        }
-
-        if (request.method == 'GET') {
+        if (request.method == 'HEAD' || request.method == 'GET') {
           // 'readAsBytes'
           var contents = await entity.readAsBytesAsyncIfPossible();
           var headers = {
             'content-length': '${contents.length}',
             ...request.headers,
           };
-          return Response.ok(contents, headers: headers);
+          return Response.ok(request.method == 'GET' ? contents : null,
+              headers: headers);
         }
       }
       return Response.notFound(path);