dart2js: add support for configuration-specific imports.

pkg/compiler/lib/src/parser/listener.dart

Index: pkg/compiler/lib/src/parser/listener.dart
diff --git a/pkg/compiler/lib/src/parser/listener.dart b/pkg/compiler/lib/src/parser/listener.dart
index 6511abceaaa4ebfa1914efc9aa2d770544e1f3c0..618b86a1b09e36fbe74d6c55cb9a28b3b251a0c7 100644
--- a/pkg/compiler/lib/src/parser/listener.dart
+++ b/pkg/compiler/lib/src/parser/listener.dart
@@ -232,6 +232,24 @@ class Listener {
                  Token asKeyword, Token semicolon) {
   }
 
+  void beginConditionalUris(Token token) {
+  }
+
+  void endConditionalUris(int count) {
+  }
+
+  void beginConditionalUri(Token ifKeyword) {
+  }
+
+  void endConditionalUri(Token ifKeyword, Token equalitySign) {
+  }
+
+  void beginDottedName(Token token) {
+  }
+
+  void endDottedName(int count, Token firstIdentifier) {
+  }
+
   void beginInitializedIdentifier(Token token) {
   }