Prefer the Dart repo's SDK to DART_SDK.

lib/src/sdk.dart

Index: lib/src/sdk.dart
diff --git a/lib/src/sdk.dart b/lib/src/sdk.dart
index 3adc9c743c414b568e38bdda9a6d5399006776d2..92e9cec251f4b202e9b3f4c90ced68896ec80e00 100644
--- a/lib/src/sdk.dart
+++ b/lib/src/sdk.dart
@@ -18,11 +18,11 @@ import 'io.dart';
 /// when building Observatory), this will be the repo's "sdk/" directory, which
 /// doesn't look exactly like the built SDK.
 final String rootDirectory = (() {
+  if (runningFromDartRepo) return p.join(dartRepoRoot, 'sdk');
+
   var dartSdk = Platform.environment["DART_SDK"];
   if (dartSdk != null) return dartSdk;
 
-  if (runningFromDartRepo) return p.join(dartRepoRoot, 'sdk');
-
   // The Dart exectuable is in "/path/to/sdk/bin/dart", so two levels up is
   // "/path/to/sdk".
   var aboveExecutable = p.dirname(p.dirname(Platform.executable));