Don't crash 'pub get' if a dev dependency is broken.

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

Index: sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart b/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
index 8fb27db00dbbb7295abc6f678224ee75cdacc425..b396c9cdd00f9aa27f5cfe8c8e09b4a98f2c934f 100644
--- a/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/pubspec.dart
@@ -100,9 +100,6 @@ class Pubspec {
   List<PackageDep> get dependencies {
     if (_dependencies != null) return _dependencies;
     _dependencies = _parseDependencies('dependencies');
-    if (_devDependencies == null) {
-      _checkDependencyOverlap(_dependencies, devDependencies);
-    }
     return _dependencies;
   }
   List<PackageDep> _dependencies;
@@ -111,9 +108,7 @@ class Pubspec {
   List<PackageDep> get devDependencies {
     if (_devDependencies != null) return _devDependencies;
     _devDependencies = _parseDependencies('dev_dependencies');
-    if (_dependencies == null) {
-      _checkDependencyOverlap(dependencies, _devDependencies);
-    }
+    _checkDependencyOverlap(dependencies, _devDependencies);
     return _devDependencies;
   }
   List<PackageDep> _devDependencies;
@@ -519,10 +514,11 @@ class Pubspec {
           pubspecPath = path.fromUri(_location);
         }
 
-        return _sources[sourceName].parseDescription(
+        var desc = _sources[sourceName].parseDescription(
             pubspecPath,
             descriptionNode.value,
             fromLockFile: false);
+        return desc;
       });
 
       dependencies.add(