Something like this should do the trick:
import 'dart:convert';
void main() {
final hex = base64Decode(
'g7DmoaMppUjtbxe5Qm9NlMr5pRitu7TayInEUMeOW+FDxYW3GAh+G0YsfmoAyfqTw72fWphF9ncT'
'P7QnsXAM8Of6o92AuVzryBt5Usm4sJ32GLaH+9SNx3J5DK1NjKrWSbP0F/EyrGjoveBc2Ut7XntM'
'p8NcGn39eD+PHQKfWnc=')
.map((e) => e.toRadixString(16).padLeft(2, '0'))
.join();
print(hex);
// 83b0e6a1a329a548ed6f17b9426f4d94caf9a518adbbb4dac889c450c78e5be143c585b718087e1b462c7e6a00c9fa93c3bd9f5a9845f677133fb427b1700cf0e7faa3dd80b95cebc81b7952c9b8b09df618b687fbd48dc772790cad4d8caad649b3f417f132ac68e8bde05cd94b7b5e7b4ca7c35c1a7dfd783f8f1d029f5a77
}
Updated solution which handles newlines in input:
import 'dart:convert';
void main() {
print(base64ToHex(
'g7DmoaMppUjtbxe5Qm9NlMr5pRitu7TayInEUMeOW+FDxYW3GAh+G0YsfmoAyfqTw72fWphF9ncT
'
'P7QnsXAM8Of6o92AuVzryBt5Usm4sJ32GLaH+9SNx3J5DK1NjKrWSbP0F/EyrGjoveBc2Ut7XntM
'
'p8NcGn39eD+PHQKfWnc='));
// 83b0e6a1a329a548ed6f17b9426f4d94caf9a518adbbb4dac889c450c78e5be143c585b718087e1b462c7e6a00c9fa93c3bd9f5a9845f677133fb427b1700cf0e7faa3dd80b95cebc81b7952c9b8b09df618b687fbd48dc772790cad4d8caad649b3f417f132ac68e8bde05cd94b7b5e7b4ca7c35c1a7dfd783f8f1d029f5a77
}
String base64ToHex(String source) =>
base64Decode(LineSplitter.split(source).join())
.map((e) => e.toRadixString(16).padLeft(2, '0'))
.join();
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