Make computeSignature private to modelx.

tests/compiler/dart2js/type_substitution_test.dart

Index: tests/compiler/dart2js/type_substitution_test.dart
diff --git a/tests/compiler/dart2js/type_substitution_test.dart b/tests/compiler/dart2js/type_substitution_test.dart
index 9c871a9696d77c2b4574332ed8c02b708e225109..91c87184c3d2d572ec0c2e803191110b12969b1e 100644
--- a/tests/compiler/dart2js/type_substitution_test.dart
+++ b/tests/compiler/dart2js/type_substitution_test.dart
@@ -16,8 +16,8 @@ DartType getType(compiler, String name) {
   var element = clazz.buildScope().lookup(name);
   Expect.isNotNull(element);
   Expect.equals(element.kind, ElementKind.FUNCTION);
-  FunctionSignature signature =
-      element.computeSignature(compiler.resolution);
+  element.computeType(compiler.resolution);
+  FunctionSignature signature = element.functionSignature;
 
   // Function signatures are used to be to provide void types (only occuring as
   // as return types) and (inlined) function types (only occuring as method