Properly check to see if dart2js is generating source maps before declaring them.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index 6e926f385cc2cc4ffbd06bb8d9266392db1b1d02..ca71b159c979583e9fa14d50877191248ecad331 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -43,6 +43,9 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
   final BuildEnvironment _environment;
   final BarbackSettings _settings;
 
+  /// Whether source maps should be generated for the compiled JS.
+  bool get _generateSourceMaps => _settings.mode != BarbackMode.RELEASE;
+
   Dart2JSTransformer.withSettings(this._environment, this._settings) {
     var invalidOptions = _settings.configuration.keys.toSet()
         .difference(_validOptions);
@@ -89,7 +92,7 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
     var primaryId = transform.primaryId;
     transform.declareOutput(primaryId.addExtension(".js"));
     transform.declareOutput(primaryId.addExtension(".precompiled.js"));
-    if (generateSourceMaps) {
+    if (_generateSourceMaps) {
       transform.declareOutput(primaryId.addExtension(".js.map"));
     }
     return new Future.value();
@@ -117,7 +120,7 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
   /// Run the dart2js compiler.
   Future _doCompilation(Transform transform) {
     var provider = new _BarbackCompilerProvider(_environment, transform,
-        generateSourceMaps: _settings.mode != BarbackMode.RELEASE);
+        generateSourceMaps: _generateSourceMaps);
 
     // Create a "path" to the entrypoint script. The entrypoint may not actually
     // be on disk, but this gives dart2js a root to resolve relative paths