Add DartSdk.languageVersion

Bug: https://github.com/dart-lang/build/issues/2763#issuecomment-666707445
Change-Id: I8232bd395abc9efea0e5c27716450d6a5442b3bb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/156489
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: William Hesse <whesse@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/testing/lib/src/analyze.dart

@@ -218,7 +218,7 @@ Future<Null> analyzeUris(
   List<String> arguments = <String>[
     "--packages=${toFilePath(packages)}",
     "--format=machine",
-    "--dart-sdk=${Uri.base.resolve('sdk/').toFilePath()}",
+    "--dart-sdk=${_findSdkPath()}",
   ];
   if (analysisOptions != null) {
     arguments.add("--options=${toFilePath(analysisOptions)}");
@@ -274,6 +274,18 @@ Future<Null> analyzeUris(
   }
 }
 
+String _findSdkPath() {
+  var executableUri = Uri.file(Platform.executable);
+  if (File.fromUri(executableUri.resolve('../version')).existsSync()) {
+    return executableUri.resolve('..').toFilePath();
+  } else if (File.fromUri(executableUri.resolve('dart-sdk/version'))
+      .existsSync()) {
+    return executableUri.resolve('dart-sdk').toFilePath();
+  } else {
+    throw StateError('Cannot find dart-sdk for $executableUri');
+  }
+}
+
 Future<String> git(String command, Iterable<String> arguments,
     {String workingDirectory}) async {
   ProcessResult result = await Process.run(