Move testing/litetest to pub workspaces. (flutter/engine#54082)

More incremental version of https://github.com/flutter/engine/pull/53997.

engine/src/flutter/pubspec.yaml

@@ -80,6 +80,7 @@ environment:
 
 # Declare all packages that are part of the workspace.
 workspace:
+  - testing/litetest
   - tools/engine_tool
 
 # Declare all dependencies that are used by one or more packages.
@@ -145,8 +146,6 @@ dependency_overrides:
     path: ./third_party/dart/pkg/expect
   file:
     path: ./third_party/dart/third_party/pkg/file/packages/file
-  litetest:
-    path: ./testing/litetest
   logging:
     path: ./third_party/dart/third_party/pkg/logging
   meta:

engine/src/flutter/shell/platform/fuchsia/dart-pkg/zircon/BUILD.gn

@@ -61,7 +61,6 @@ dart_library("zircon_lib") {
 dart_library("zircon_tests_lib") {
   testonly = true
   package_name = "zircon_tests"
-  language_version = "2.12"
   source_dir = "."
   package_root = "test"
 

engine/src/flutter/testing/litetest/pubspec.yaml

@@ -5,29 +5,15 @@
 name: litetest
 publish_to: none
 
-# Do not add any dependencies that require more than what is provided in
-# //third_party/dart/pkg or //third_party/dart/third_party/pkg.
-# In particular, package:test is not usable here.
-
-# If you do add packages here, make sure you can run `pub get --offline`, and
-# check the .packages and .package_config to make sure all the paths are
-# relative to this directory into //third_party/dart
-
+# Required for workspace support.
 environment:
-  sdk: '>=3.2.0-0 <4.0.0'
+  sdk: ^3.5.0-294.0.dev
+
+# This package is managed as part of the engine workspace.
+resolution: workspace
 
 dependencies:
   async_helper: any
   expect: any
   meta: any
   smith: any
-
-dependency_overrides:
-  async_helper:
-    path: ../../third_party/dart/pkg/async_helper
-  expect:
-    path: ../../third_party/dart/pkg/expect
-  meta:
-    path: ../../third_party/dart/pkg/meta
-  smith:
-    path: ../../third_party/dart/pkg/smith

engine/src/flutter/tools/fuchsia/dart/gen_dart_package_config.py

@@ -49,10 +49,15 @@ def language_version_from_pubspec(pubspec):
     if not parsed:
       return DEFAULT_LANGUAGE_VERSION
 
-    # If a format like sdk: '>=a.b' or sdk: 'a.b' is found, we'll use a.b.
+    # If any format like:
+    #   sdk: '>=a.b'
+    #   sdk: '^a.b'
+    #   sdk: 'a.b'
+    # ... is found, we 'a.b' as the language version.
+    #
     # In all other cases we default to "2.8"
     env_sdk = parsed.get('environment', {}).get('sdk', 'any')
-    match = re.search(r'^(>=)?((0|[1-9]\d*)\.(0|[1-9]\d*))', env_sdk)
+    match = re.search(r'^(>=|\^)?((0|[1-9]\d*)\.(0|[1-9]\d*))', env_sdk)
     if match:
       min_sdk_version = match.group(2)
     else: