Allow returning from a sourced script.

Fixes issue #53.

core/cmd_exec.py

@@ -197,6 +197,11 @@ def _Source(self, argv):
        line_reader = reader.FileLineReader(f, self.arena)
        _, c_parser = parse_lib.MakeParser(line_reader, self.arena)
        return self._EvalHelper(c_parser, path)
    except _ControlFlow as e:
      if e.IsReturn():
        return e.ReturnValue()
      else:
        raise
    except IOError as e:
      # TODO: Should point to the source statement that failed.
      util.error('source %r failed: %s', path, os.strerror(e.errno))

spec/builtins.test.sh

@@ -95,6 +95,13 @@ echo status=$?
# OK bash stdout: status=2
# OK dash stdout: status=127

### Source script that returns
echo one
. spec/return-helper.sh
echo $?
echo two
# stdout-json: "one\nreturn-helper.sh\n42\ntwo\n"

### Exit builtin
exit 3
# status: 3

spec/return-helper.sh

@@ -0,0 +1,5 @@

echo 'return-helper.sh'
return 42
echo 'should not see'