[ddc] Fix semantics of capturing loop variables

Fixes the for loop lowering to JavaScript when there are function
expressions in the loop initialization, condition, or updates.

Manually assign source mapping information on js_ast nodes to 
make stepping in the Dart debugger make as much sense as possible.
With these mappings three steps are required on each loop variable 
on each iteration but it ensures that the mapping always points to the
variable instead of some random line of code in or around the original
for loop.

Fixes the failure on language/loop/for_variable_capture_test.

Issue: https://github.com/dart-lang/language/issues/3539
Change-Id: I755b88faef11804986fb3d1b2c328da3dd08fe8d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/345145
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>