Add flag to measure impact of type check elimination.

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
===================================================================
--- runtime/vm/flow_graph_builder.cc	(revision 7822)
+++ runtime/vm/flow_graph_builder.cc	(working copy)
@@ -20,9 +20,11 @@
 
 namespace dart {
 
+DEFINE_FLAG(bool, eliminate_type_checks, true,
+            "Eliminate type checks when allowed by static type analysis");
+DEFINE_FLAG(bool, print_ast, false, "Print abstract syntax tree.");
 DEFINE_FLAG(bool, print_flow_graph, false, "Print the IR flow graph.");
 DECLARE_FLAG(bool, enable_type_checks);
-DEFINE_FLAG(bool, print_ast, false, "Print abstract syntax tree.");
 
 
 FlowGraphBuilder::FlowGraphBuilder(const ParsedFunction& parsed_function)
@@ -311,6 +313,9 @@
   ASSERT(FLAG_enable_type_checks);
   ASSERT(!dst_type.IsNull());
   ASSERT(dst_type.IsFinalized());
+  if (!FLAG_eliminate_type_checks) {
+    return false;
+  }
 
   // Any expression is assignable to the Dynamic type and to the Object type.
   // Skip the test.