[bug] assigning using := prints during source( , echo=TRUE) (but not from console)

[jan-glx]

> dt <- data.table(a=1)
> dt[,a:=1]
> source('~/.active-rstudio-document', echo=TRUE)

> dt <- data.table(a=1)

> dt[,a:=1]
   a
1: 1

Related to #2319.

sessionInfo():

R version 3.4.1 RC (2017-06-22 r72854)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows >= 8 x64 (build 9200)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252 LC_NUMERIC=C                           LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.10.5

loaded via a namespace (and not attached):
[1] compiler_3.4.1 tools_3.4.1   

[MichaelChirico]

Confirming this is not RStudio-specific; command line code:

echo "library(data.table)
DT = data.table(a = 1)
DT[ , a := 1]
" > test.R
R
source('test.R', echo = TRUE)
#    a
# 1: 1
DT[ , a := 1]

Also related to #933