VM: Optimized context allocation on all platforms.

runtime/vm/flow_graph_compiler.h

Index: runtime/vm/flow_graph_compiler.h
===================================================================
--- runtime/vm/flow_graph_compiler.h	(revision 38897)
+++ runtime/vm/flow_graph_compiler.h	(working copy)
@@ -190,9 +190,15 @@
   Label* entry_label() { return &entry_label_; }
   Label* exit_label() { return &exit_label_; }
 
+  void GenerateCode(FlowGraphCompiler* compiler) {
+    ASSERT(exit_label_.IsBound());
+    EmitNativeCode(compiler);
+    ASSERT(entry_label_.IsBound());
+  }
+
+ private:
   virtual void EmitNativeCode(FlowGraphCompiler* compiler) = 0;
 
- private:
   Label entry_label_;
   Label exit_label_;