This change is similar to:
* https://github.com/flutter/flutter/issues/82728
* https://github.com/flutter/flutter/issues/87073

We are using a more recent Chromium ICU fork commit.

See for reference: https://chromium.googlesource.com/chromium/deps/icu

Flutter needs to use a reasonably recent version of the ICU library. The
Chromium's fork of ICU, which is in use in Flutter as well, has taken in
a few updates since the initial 69.1 release.

Update the DEPS file in github.com/flutter/engine to use the commit ID
`a38aef9142ace942a8bf166020c569f4cda0f8d3` for the ICU library.

This takes in recent patches to the ICU library, most relevant of which
is a fix for Norwegian language support.

Fixes: #flutter/flutter/88600

See https://github.com/flutter/engine/pull/27920

This contains a fix needed to build with the latest Clang toolchain.

* Roll dart to 2.15.0-18.0.dev

Changes since last roll:
```
d39206fb4e2 (tag: 2.15.0-18.0.dev) Version 2.15.0-18.0.dev
6e28f8bb404 [vm] Avoid reserved register error on newer compilers
d2bd43f43e6 [python3] Migrate PRESUBMIT.py files
79be5898b46 [co19] LibTests/typed_data tests skipped for dart2js
af6f6cf9826 (tag: 2.15.0-17.0.dev) Version 2.15.0-17.0.dev
f0f94705a49 In ScopeResolverVisitor, don't visit identifiers to the right of `.`.
6274633ea1b Merge ScopedVisitor with ScopeResolverVisitor classes.
80ec62a3fd2 (tag: 2.15.0-16.0.dev) Version 2.15.0-16.0.dev
fc4d4144fb0 (origin/base) Added js_util import to html_dart2js
d2855e703e2 [ CLI ] Default to printing CLI usage message instead of VM usage
af2ec43d447 [vm/compiler] Fix incorrect assertion in LoadOptimizer
3365b77ac2d use code from package:devtools_shared
16ff4aec0ea Revert "[vm] Hide internal implementation List types and expose them as List"
6465ee4ad8a analyze pkg/nnbd_migration on the bots
cd6fb671034 [vm] Clean up changes around internal-only class ids.
7ac9c46977b [dart2js_info] Fix tests after migration.
1f405bf163c [VM/Runtime] Added a Dart VM initialization state to prevent racy access              to VM state from Dart API calls.
824bec596f5 [vm] Hide internal implementation List types and expose them as List
```