Get rid of DART_SDK in pub.

sdk/lib/_internal/pub/test/test_pub.dart

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index 33ea60ea3e196e3de03ff42eba03622836a17166..59be37c7007921508e8c66b61496d8e51a52f94a 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -248,10 +248,6 @@ String _sandboxDir;
 /// sandbox directory.
 final String cachePath = "cache";
 
-/// The path of the mock SDK directory used for tests. Relative to the sandbox
-/// directory.
-final String sdkPath = "sdk";
-
 /// The path of the mock app directory used for tests. Relative to the sandbox
 /// directory.
 final String appPath = "myapp";
@@ -340,11 +336,6 @@ void _integration(String description, void body(), [Function testFn]) {
       currentSchedule.timeout = new Duration(seconds: 10);
     }
 
-    // Ensure the SDK version is always available.
-    d.dir(sdkPath, [
-      d.file('version', '0.1.2.3')
-    ]).create();
-
     _sandboxDir = createSystemTempDir();
     d.defaultRoot = sandboxDir;
     currentSchedule.onComplete.schedule(() => deleteEntry(_sandboxDir),
@@ -483,7 +474,10 @@ ScheduledProcess startPub({List args, Future<Uri> tokenEndpoint}) {
     var environment = {};
     environment['_PUB_TESTING'] = 'true';
     environment['PUB_CACHE'] = pathInSandbox(cachePath);
-    environment['DART_SDK'] = pathInSandbox(sdkPath);
+
+    // Ensure a known SDK version is set for the tests that rely on that.
+    environment['_PUB_TEST_SDK_VERSION'] = "0.1.2+3";
+
     if (tokenEndpoint != null) {
       environment['_PUB_TEST_TOKEN_ENDPOINT'] =
         tokenEndpoint.toString();