mojo_client: Dont try to iterate over response headers if there aren't any. (#3811)

packages/flutter/lib/src/http/mojo_client.dart

@@ -180,11 +180,13 @@ class MojoClient {
       ByteData data = await mojo.DataPipeDrainer.drainHandle(response.body);
       Uint8List bodyBytes = new Uint8List.view(data.buffer);
       Map<String, String> headers = <String, String>{};
+      if (response.headers != null) {
         for (mojom.HttpHeader header in response.headers) {
           String headerName = header.name.toLowerCase();
           String existingValue = headers[headerName];
           headers[headerName] = existingValue != null ? '$existingValue, ${header.value}' : header.value;
         }
+      }
       return new Response.bytes(bodyBytes, response.statusCode, headers: headers);
     } catch (exception, stack) {
       FlutterError.reportError(new FlutterErrorDetails(