[analysis_server] Fix compilation of server when using custom SDK

When running analysis server tests from source, we now (since a recent change) pre-compile the server to avoid compilation on every test.

If the first test run was `test\integration\analysis\get_errors_non_standard_sdk_test.dart` we would try to compile with the non-standard (and incomplete) SDK created by the test, and fail because of #52752. This changes to always use the real SDK that we're running with for the compilation instead.

Change-Id: I1028c24ecb18f9caa465b579866d07a72c1dc02b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/433822
Reviewed-by: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: DanTup

pkg/analysis_server/test/support/sdk_paths.dart

@@ -45,7 +45,9 @@ String get sdkRootPath {
 /// When running from source, the server will be compiled to disk for the first
 /// request to speed up subsequent tests in the same process.
 Future<String> getAnalysisServerPath(String dartSdkPath) async {
-  var dartBinary = path.join(dartSdkPath, 'bin', 'dart');
+  // Always use the "real" SDK binary for compilation, not the path provided,
+  // which might be an incomplete SDK that is the target of the test.
+  var dartBinary = Platform.resolvedExecutable;
   var snapshotPath = path.join(
     dartSdkPath,
     'bin',