Fix a stack overflow when loading transformers.

lib/src/barback/dependency_computer.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.barback.dependency_computer;
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
 
 import '../dart.dart';
@@ skipping 269 matching lines @@
             // Probe [id]'s transformer dependencies to ensure that it doesn't
             // depend on this package. If it does, a CycleError will be thrown.
             _dependencyComputer._transformersNeededByTransformer(id);
           } else {
             // Store the transformers needed specifically with the current set
             // of [_applicableTransformers]. When reporting this transformer's
             // dependencies, [computeTransformersNeededByTransformers] will use
             // this stored set of dependencies rather than the potentially wider
             // set that would be recomputed if [transformersNeededByLibrary]
             // were called anew.
-            _transformersNeededByTransformers[id] =
-                transformersNeededByLibrary(_package.transformerPath(id));
+            _transformersNeededByTransformers.putIfAbsent(id, () =>
+                transformersNeededByLibrary(_package.transformerPath(id)));
           }
         } on CycleException catch (error) {
           throw error.prependStep("$packageName is transformed by $id");
         }
 
         return true;
       }));
 
       // Clear the cached imports and exports because the new transformers may
       // start transforming a library whose directives were previously
@@ skipping 142 matching lines @@
       return null;
     }
 
     _directives[libraryUri] =
         parseImportsAndExports(readTextFile(library), name: library)
         .map((directive) => Uri.parse(directive.uri.stringValue))
         .toSet();
     return _directives[libraryUri];
   }
 }