Revert "[gn] Consolidate exec_script calls to speed up generation"

Revert "[gn] Consolidate exec_script calls to speed up generation"

Appears to break builds with errors like:

ninja -C xcodebuild/ProductX64 runtime
ninja: Entering directory `xcodebuild/ProductX64'
ninja: error: '../../runtime/vm/async_patch.dart', needed by 'obj/runtime/vm/concatenate_async_patch.inputdeps.stamp', missing and no known rule to make it
BUILD FAILED
step returned non-zero exit code: 1
@@@STEP_FAILURE@@@

This reverts commit ad86d6ed26e3b6eeeaa96b654881a49c4b04a561.

BUG=

Committed: https://github.com/dart-lang/sdk/commit/e18404fd3b7fc907600fce68e6cbbf0ed7415947

[jamesr, 2016-11-10 23:17:28]

Description was changed from

==========
Revert "[gn] Consolidate exec_script calls to speed up generation"

Appears to break builds with errors like:

ninja -C xcodebuild/ProductX64 runtime
ninja: Entering directory `xcodebuild/ProductX64'
ninja: error: '../../runtime/vm/async_patch.dart', needed by
'obj/runtime/vm/concatenate_async_patch.inputdeps.stamp', missing and no known
rule to make it
BUILD FAILED
step returned non-zero exit code: 1
@@@STEP_FAILURE@@@

This reverts commit ad86d6ed26e3b6eeeaa96b654881a49c4b04a561.

BUG=
==========

to

==========
Revert "[gn] Consolidate exec_script calls to speed up generation"

Appears to break builds with errors like:

ninja -C xcodebuild/ProductX64 runtime
ninja: Entering directory `xcodebuild/ProductX64'
ninja: error: '../../runtime/vm/async_patch.dart', needed by
'obj/runtime/vm/concatenate_async_patch.inputdeps.stamp', missing and no known
rule to make it
BUILD FAILED
step returned non-zero exit code: 1
@@@STEP_FAILURE@@@

This reverts commit ad86d6ed26e3b6eeeaa96b654881a49c4b04a561.

BUG=

Committed:
https://github.com/dart-lang/sdk/commit/e18404fd3b7fc907600fce68e6cbbf0ed7415947
==========

[jamesr, 2016-11-10 23:17:29]

Committed patchset #1 (id:1) manually as
e18404fd3b7fc907600fce68e6cbbf0ed7415947 (presubmit successful).