Change fromString constructor to parse static method

sdk/lib/_internal/pub/lib/src/http.dart

Index: sdk/lib/_internal/pub/lib/src/http.dart
diff --git a/sdk/lib/_internal/pub/lib/src/http.dart b/sdk/lib/_internal/pub/lib/src/http.dart
index 123c20601cae3dd61dfce2ab03c16362308d7e5a..eecfda3fcdd86b5435fde55a418df65aa1688e6c 100644
--- a/sdk/lib/_internal/pub/lib/src/http.dart
+++ b/sdk/lib/_internal/pub/lib/src/http.dart
@@ -97,7 +97,7 @@ class PubHttpClient extends http.BaseClient {
     if (request.method == 'POST') {
       var contentTypeString = request.headers[HttpHeaders.CONTENT_TYPE];
       if (contentTypeString == null) contentTypeString = '';
-      var contentType = new ContentType.fromString(contentTypeString);
+      var contentType = ContentType.parse(contentTypeString);
       if (request is http.MultipartRequest) {
         requestLog.writeln();
         requestLog.writeln("Body fields:");