Rename the global inference task and reduce it's API surface by introducing the

tests/compiler/dart2js/type_inference6_test.dart

Index: tests/compiler/dart2js/type_inference6_test.dart
diff --git a/tests/compiler/dart2js/type_inference6_test.dart b/tests/compiler/dart2js/type_inference6_test.dart
index bfee36c7ebdd67af4f6ae34fbf518dc5d4f253dd..7eaf0a5fe6b9333c15944bc5506c5a9dd8360bee 100644
--- a/tests/compiler/dart2js/type_inference6_test.dart
+++ b/tests/compiler/dart2js/type_inference6_test.dart
@@ -24,11 +24,11 @@ Future runTest() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   return compiler.run(uri).then((_) {
-    var typesTask = compiler.typesTask;
-    var typesInferrer = typesTask.typesInferrer;
+    var commonMasks = compiler.commonMasks;
+    var typesInferrer = compiler.globalInference.typesInferrer;
     var element = findElement(compiler, "foo");
     var mask = typesInferrer.getReturnTypeOfElement(element);
-    Expect.equals(typesTask.uint31Type, simplify(mask, compiler));
+    Expect.equals(commonMasks.uint31Type, simplify(mask, compiler));
   });
 }