Add HasSubclasses function to class hierarchy analysis.

runtime/vm/class_table_test.cc

Index: runtime/vm/class_table_test.cc
===================================================================
--- runtime/vm/class_table_test.cc	(revision 11007)
+++ runtime/vm/class_table_test.cc	(working copy)
@@ -120,6 +120,11 @@
       class_table->GetOverridesOf(class_a_bar);
   EXPECT_EQ(1, class_a_bar_overrides->length());
   EXPECT_EQ(class_d_bar.raw(), (*class_a_bar_overrides)[0]->raw());
+
+  EXPECT(class_table->HasSubclasses(class_a_id));
+  EXPECT(class_table->HasSubclasses(class_b_id));
+  EXPECT(!class_table->HasSubclasses(class_c_id));
+  EXPECT(!class_table->HasSubclasses(class_d_id));
 }
 
 }  // namespace dart