Add 'withKeyword' to mixin application with Fasta.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

Index: pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
index ea08eaf32f016713be51daca112e2495eba608d6..993d200dabbf42a2e795af9cd04c20875b25b6be 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -1056,12 +1056,9 @@ class AstBuilder extends ScopeListener {
   }
 
   @override
-  void endMixinApplication() {
+  void endMixinApplication(Token withKeyword) {
     debugEvent("MixinApplication");
     List<TypeName> mixinTypes = pop();
-    // TODO(paulberry,ahe): the parser doesn't give us enough information to
-    // locate the "with" keyword.
-    Token withKeyword;
     TypeName supertype = pop();
     push(new _MixinApplication(supertype, withKeyword, mixinTypes));
   }