Implement external strings.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index b6d7b9769417123545fe523db78b5bc9263c5848..b45aa6afdd7cb7cda3770aba031502164682d9ea 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -49,6 +49,9 @@ namespace dart {
       V(OneByteString)                                                         \
       V(TwoByteString)                                                         \
       V(FourByteString)                                                        \
+      V(ExternalOneByteString)                                                 \
+      V(ExternalTwoByteString)                                                 \
+      V(ExternalFourByteString)                                                \
     V(Bool)                                                                    \
     V(Array)                                                                   \
       V(ImmutableArray)                                                        \
@@ -687,6 +690,30 @@ class RawFourByteString : public RawString {
 };
 
 
+class RawExternalOneByteString : public RawString {
+  RAW_HEAP_OBJECT_IMPLEMENTATION(ExternalOneByteString);
+
+  uint8_t* data_;
+  void* peer_;
+};
+
+
+class RawExternalTwoByteString : public RawString {
+  RAW_HEAP_OBJECT_IMPLEMENTATION(ExternalTwoByteString);
+
+  uint16_t* data_;
+  void* peer_;
+};
+
+
+class RawExternalFourByteString : public RawString {
+  RAW_HEAP_OBJECT_IMPLEMENTATION(ExternalFourByteString);
+
+  uint32_t* data_;
+  void* peer_;
+};
+
+
 class RawBool : public RawInstance {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Bool);