Add Dart_IsExternalString and Dart_ExternalStringGetPeer to the dart

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 2079)
+++ runtime/vm/object.h	(working copy)
@@ -2475,6 +2475,12 @@
 
   bool IsSymbol() const;
 
+  virtual bool IsExternal() const { return false; }
+  virtual void* GetPeer() const {
+    UNREACHABLE();
+    return NULL;
+  }
+
   static RawString* New(const char* str, Heap::Space space = Heap::kNew);
   static RawString* New(const uint8_t* characters,
                         intptr_t len,
@@ -2751,6 +2757,11 @@
     return kOneByteChar;
   }
 
+  virtual bool IsExternal() const { return true; }
+  virtual void* GetPeer() const {
+    return raw_ptr()->external_data_->peer_;
+  }
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawExternalOneByteString));
   }
@@ -2788,6 +2799,11 @@
     return kTwoByteChar;
   }
 
+  virtual bool IsExternal() const { return true; }
+  virtual void* GetPeer() const {
+    return raw_ptr()->external_data_->peer_;
+  }
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawExternalTwoByteString));
   }
@@ -2825,6 +2841,11 @@
     return kFourByteChar;
   }
 
+  virtual bool IsExternal() const { return true; }
+  virtual void* GetPeer() const {
+    return raw_ptr()->external_data_->peer_;
+  }
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawExternalFourByteString));
   }