Prefer the Dart repo's SDK to DART_SDK.

lib/src/sdk.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Operations relative to the user's installed Dart SDK.
 library pub.sdk;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 
 import 'io.dart';
 
 /// The path to the root directory of the SDK.
 ///
 /// Note that if pub is running from source within the Dart repo (for example
 /// 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));
   assert(fileExists(p.join(aboveExecutable, 'version')));
   return aboveExecutable;
 })();
 
 /// The SDK's revision number formatted to be a semantic version.
 ///
 /// This can be set so that the version solver tests can artificially select
@@ skipping 34 matching lines @@
   var version = "$major.$minor.$patch";
   if (channel == "be") {
     // TODO(rnystrom): tools/utils.py includes the svn commit here. Should we?
     version += "-edge";
   } else if (channel == "dev") {
     version += "-dev.$prerelease.$prereleasePatch";
   }
 
   return new Version.parse(version);
 }