Handle circular dependencies on the root package when checking SDK constraints.

BUG=dartbug.com/8364

Review URL: https://codereview.chromium.org//12211061

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@18198 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/entrypoint.dart

@@ -157,13 +157,19 @@ class Entrypoint {
           var id = lockFile.packages[ref.name];
 
           visited.add(ref.name);
-          var future = cache.describe(id);
+          var future;
+          if (ref.name == root.name) {
+            future = new Future<Pubspec>.immediate(root.pubspec);
+          } else {
+            future = cache.describe(id);
+          }
           group.add(future.then(visitPackage));
         }
 
         return pubspec;
       }
 
+      visited.add(root.name);
       visitPackage(root.pubspec);
       return group.future;
     });

lib/src/system_cache.dart

@@ -63,6 +63,8 @@ class SystemCache {
   /// Gets the package identified by [id]. If the package is already cached,
   /// reads it from the cache. Otherwise, requests it from the source.
   Future<Pubspec> describe(PackageId id) {
+    if (id.isRoot) throw new ArgumentError("Cannot describe the root package.");
+
     // Try to get it from the system cache first.
     if (id.source.shouldCache) {
       return id.systemCacheDirectory.then((packageDir) {

test/sdk_constraint_test.dart

@@ -117,5 +117,40 @@ main() {
             or adding a version constraint to use an older version of a package.
             """);
     });
+
+    integration("handles a circular dependency on the root package", () {
+      // Using an SDK source, but this should be true of all sources.
+      dir(sdkPath, [
+        dir("pkg", [
+          dir("foo", [
+            libPubspec("foo", "0.0.1", sdk: ">3.0.0", deps: [
+              {"sdk": "myapp"}
+            ]),
+            libDir("foo")
+          ])
+        ])
+      ]).scheduleCreate();
+
+      dir(appPath, [
+        pubspec({
+          "name": "myapp",
+          "dependencies": {
+            "foo": { "sdk": "foo" }
+          },
+          "environment": {"sdk": ">2.0.0"}
+        })
+      ]).scheduleCreate();
+
+      schedulePub(args: [command],
+          error:
+            """
+            Some packages are not compatible with your SDK version 0.1.2+3:
+            - 'myapp' requires >2.0.0
+            - 'foo' requires >3.0.0
+
+            You may be able to resolve this by upgrading to the latest Dart SDK
+            or adding a version constraint to use an older version of a package.
+            """);
+    });
   }
 }