Ignore the DART_SDK variable in pub.

Now that Platform.resolvedExecutable exists, the environment variable
only provided a way to load files from the wrong SDK.

Closes #1288

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1220553002.

lib/src/sdk.dart

@@ -20,12 +20,9 @@ import 'io.dart';
 final String rootDirectory = (() {
   if (runningFromDartRepo) return p.join(dartRepoRoot, 'sdk');
 
-  var dartSdk = Platform.environment["DART_SDK"];
-  if (dartSdk != null) return dartSdk;
-
   // 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));
+  var aboveExecutable = p.dirname(p.dirname(Platform.resolvedExecutable));
   assert(fileExists(p.join(aboveExecutable, 'version')));
   return aboveExecutable;
 })();

pubspec.yaml

@@ -36,4 +36,4 @@ dev_dependencies:
   unittest: "^0.11.0"
 
 environment:
-  sdk: ">=1.11.0-dev.3.0 <2.0.0"
+  sdk: ">=1.11.0 <2.0.0"