Add support for debug break in Dart source.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
===================================================================
--- runtime/vm/intermediate_language.h	(revision 45190)
+++ runtime/vm/intermediate_language.h	(working copy)
@@ -440,6 +440,7 @@
   M(Return)                                                                    \
   M(Throw)                                                                     \
   M(ReThrow)                                                                   \
+  M(Stop)                                                                      \
   M(Goto)                                                                      \
   M(IndirectGoto)                                                              \
   M(Branch)                                                                    \
@@ -2125,6 +2126,32 @@
 };
 
 
+class StopInstr : public TemplateInstruction<0, NoThrow> {
+ public:
+  explicit StopInstr(const char* message)
+      : message_(message) {
+    ASSERT(message != NULL);
+  }
+
+  const char* message() const { return message_; }
+
+  DECLARE_INSTRUCTION(Stop);
+
+  virtual intptr_t ArgumentCount() const { return 0; }
+
+  virtual bool CanDeoptimize() const { return false; }
+
+  virtual EffectSet Effects() const { return EffectSet::None(); }
+
+  virtual EffectSet Dependencies() const { return EffectSet::None(); }
+
+ private:
+  const char* message_;
+
+  DISALLOW_COPY_AND_ASSIGN(StopInstr);
+};
+
+
 class GotoInstr : public TemplateInstruction<0, NoThrow> {
  public:
   explicit GotoInstr(JoinEntryInstr* entry)