Add support for debug break in Dart source.

runtime/vm/ast.h

Index: runtime/vm/ast.h
===================================================================
--- runtime/vm/ast.h	(revision 45190)
+++ runtime/vm/ast.h	(working copy)
@@ -34,6 +34,7 @@
   V(DoWhile)                                                                   \
   V(For)                                                                       \
   V(Jump)                                                                      \
+  V(Stop)                                                                      \
   V(ArgumentList)                                                              \
   V(Array)                                                                     \
   V(Closure)                                                                   \
@@ -1143,6 +1144,27 @@
 };
 
 
+class StopNode : public AstNode {
+ public:
+  StopNode(intptr_t token_pos, const char* message)
+      : AstNode(token_pos),
+        message_(message) {
+    ASSERT(message != NULL);
+  }
+
+  const char* message() const { return message_; }
+
+  virtual void VisitChildren(AstNodeVisitor* visitor) const { }
+
+  DECLARE_COMMON_NODE_FUNCTIONS(StopNode);
+
+ private:
+  const char* message_;
+
+  DISALLOW_IMPLICIT_CONSTRUCTORS(StopNode);
+};
+
+
 class LoadLocalNode : public AstNode {
  public:
   LoadLocalNode(intptr_t token_pos, const LocalVariable* local)