You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: data_source_test.go
    path of file: ./repos/chainlink/core/services/llo
    
    the code of the file until where you have to start completion: package llo

import (
	"context"
	"er
    
func Test_DataSource(t *testing.T) {
	lggr := logger.TestLogger(t)
	reg := &mockRegistry{make(map[streams.StreamID]*mockStream)}
	ds := newDataSource(lggr, reg)
	ctx := testutils.Context(t)

	streamIDs := make(map[streams.StreamID]struct{})
	streamIDs[streams.StreamID(1)] = struct{}{}
	streamIDs[streams.StreamID(2)] = struct{}{}
	streamIDs[streams.StreamID(3)] = struct{}{}

	t.Run("Observe", func(t *testing.T) {
		t.Run("returns errors if no streams are defined", func(t *testing.T) {
			vals, err := ds.Observe(ctx, streamIDs)
			assert.NoError(t, err)

			assert.Equal(t, llo.StreamValues{
				2: llo.ObsResult[*big.Int]{Val: nil, Valid: false},
				1: llo.ObsResult[*big.Int]{Val: nil, Valid: false},
				3: llo.ObsResult[*big.Int]{Val: nil, Valid: false},
			}, vals)
		})
		t.Run("observes each stream with success and returns values matching map argument", func(t *testing.T) {
			reg.streams[1] = makeStreamWithSingleResult[*big.Int](big.NewInt(2181), nil)
			reg.streams[2] = makeStreamWithSingleResult[*big.Int](big.NewInt(40602), nil)
			reg.streams[3] = makeStreamWithSingleResult[*big.Int](big.NewInt(15), nil)

			vals, err := ds.Observe(ctx, streamIDs)
			assert.NoError(t, err)

			assert.Equal(t, llo.StreamValues{
				2: llo.ObsResult[*big.Int]{Val: big.NewInt(40602), Valid: true},
				1: llo.ObsResult[*big.Int]{Val: big.NewInt(2181), Valid: true},
				3: llo.ObsResult[*big.Int]{Val: big.NewInt(15), Valid: true},
			}, vals)
		})
		t.Run("observes each stream and returns success/errors", func(t *testing.T) {
			reg.streams[1] = makeStreamWithSingleResult[*big.Int](big.NewInt(2181), errors.New("something exploded"))
			reg.streams[2] = makeStreamWithSingleResult[*big.Int](big.NewInt(40602), nil)
			reg.streams[3] = makeStreamWithSingleResult[*big.Int](nil, errors.New("something exploded 2"))

			vals, err := ds.Observe(ctx, streamIDs)
			assert.NoError(t, err)

			assert.Equal(t, llo.StreamValues{
				2: llo.ObsResult[*big.Int]{Val: big.NewInt(40602), Valid: true},
				1: llo.ObsResult[*big.Int]{Val: nil, Valid: false},
				3: llo.ObsResult[*big.Int]{Val: nil, Valid: false},
			}, vals)
		})
	})
}