Don't allow packages to depend on themselves.

sdk/lib/_internal/pub/lib/src/pubspec.dart

Index: sdk/lib/_internal/pub/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub/lib/src/pubspec.dart b/sdk/lib/_internal/pub/lib/src/pubspec.dart
index 6da96ba621cc6f2456a9a5bc41e00cc99bd0ac71..049857754de00f3999f1e7e5cf8e6b19e4ab9e6a 100644
--- a/sdk/lib/_internal/pub/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub/lib/src/pubspec.dart
@@ -135,10 +135,10 @@ Pubspec _parseMap(String filePath, Map map, SourceRegistry sources) {
     version = new Version.parse(map['version']);
   }
 
-  var dependencies = _parseDependencies(filePath, sources,
+  var dependencies = _parseDependencies(name, filePath, sources,
       map['dependencies']);
 
-  var devDependencies = _parseDependencies(filePath, sources,
+  var devDependencies = _parseDependencies(name, filePath, sources,
       map['dev_dependencies']);
 
   // Make sure the same package doesn't appear as both a regular and dev
@@ -241,8 +241,8 @@ Pubspec _parseMap(String filePath, Map map, SourceRegistry sources) {
       environment, map);
 }
 
-List<PackageDep> _parseDependencies(String pubspecPath, SourceRegistry sources,
-    yaml) {
+List<PackageDep> _parseDependencies(String packageName, String pubspecPath,
+    SourceRegistry sources, yaml) {
   var dependencies = <PackageDep>[];
 
   // Allow an empty dependencies key.
@@ -255,6 +255,10 @@ List<PackageDep> _parseDependencies(String pubspecPath, SourceRegistry sources,
   }
 
   yaml.forEach((name, spec) {
+    if (name == packageName) {
+      throw new FormatException("Package '$name' cannot depend on itself.");

[nweiz, 2013/08/22 00:18:21]

Rather than specifically including the package name for this error, perhaps we
should catch all FormatExceptions and add the pubspec name, or include it
manually in each one?

[Bob Nystrom, 2013/08/22 18:09:30]

When one of these exceptions is thrown, the output is like:

Error parsing path/to/pubspec.yaml:
<error message here>

so the offending package is usually obvious. I thought putting the name
explicitly in this one made sense since it's the actual name that's the cause of
the error.

+    }
+
     var description;
     var sourceName;