Added following dart API changes to enable more efficient access based

runtime/vm/dart_api_impl_test.cc

Index: runtime/vm/dart_api_impl_test.cc
===================================================================
--- runtime/vm/dart_api_impl_test.cc	(revision 26045)
+++ runtime/vm/dart_api_impl_test.cc	(working copy)
@@ -542,6 +542,16 @@
   intptr_t len = -1;
   EXPECT_VALID(Dart_StringLength(latin1str, &len));
   EXPECT_EQ(4, len);
+  intptr_t char_size;
+  intptr_t str_len;
+  void* peer;
+  EXPECT_VALID(Dart_StringGetProperties(latin1str,
+                                        &char_size,
+                                        &str_len,
+                                        &peer));
+  EXPECT_EQ(1, char_size);
+  EXPECT_EQ(4, str_len);
+  EXPECT(!peer);
 
   uint8_t data8[] = { 'o', 'n', 'e', 0x7F };
 
@@ -562,10 +572,17 @@
   }
 
   Dart_Handle ext8 = Dart_NewExternalLatin1String(data8, ARRAY_SIZE(data8),
-                                                  NULL, NULL);
+                                                  data8, NULL);
   EXPECT_VALID(ext8);
   EXPECT(Dart_IsString(ext8));
   EXPECT(Dart_IsExternalString(ext8));
+  EXPECT_VALID(Dart_StringGetProperties(ext8,
+                                        &char_size,
+                                        &str_len,
+                                        &peer));
+  EXPECT_EQ(1, char_size);
+  EXPECT_EQ(4, str_len);
+  EXPECT_EQ(data8, peer);
 
   uint16_t data16[] = { 't', 'w', 'o', 0xFFFF };
 
@@ -574,12 +591,26 @@
   EXPECT(Dart_IsString(str16));
   EXPECT(!Dart_IsStringLatin1(str16));
   EXPECT(!Dart_IsExternalString(str16));
+  EXPECT_VALID(Dart_StringGetProperties(str16,
+                                        &char_size,
+                                        &str_len,
+                                        &peer));
+  EXPECT_EQ(2, char_size);
+  EXPECT_EQ(4, str_len);
+  EXPECT(!peer);
 
   Dart_Handle ext16 = Dart_NewExternalUTF16String(data16, ARRAY_SIZE(data16),
-                                                  NULL, NULL);
+                                                  data16, NULL);
   EXPECT_VALID(ext16);
   EXPECT(Dart_IsString(ext16));
   EXPECT(Dart_IsExternalString(ext16));
+  EXPECT_VALID(Dart_StringGetProperties(ext16,
+                                        &char_size,
+                                        &str_len,
+                                        &peer));
+  EXPECT_EQ(2, char_size);
+  EXPECT_EQ(4, str_len);
+  EXPECT_EQ(data16, peer);
 
   int32_t data32[] = { 'f', 'o', 'u', 'r', 0x10FFFF };
 
@@ -7105,8 +7136,15 @@
   Dart_EnterScope();
   Dart_Handle str = Dart_GetNativeArgument(args, 0);
   EXPECT(Dart_IsString(str));
+  void* peer;
+  Dart_Handle str_arg = Dart_GetNativeStringArgument(args, 0, &peer);
+  EXPECT(Dart_IsString(str_arg));
+  EXPECT(!peer);
   intptr_t size = 0;
   EXPECT_VALID(Dart_StringStorageSize(str, &size));
+  intptr_t arg_size = 0;
+  EXPECT_VALID(Dart_StringStorageSize(str_arg, &arg_size));
+  EXPECT_EQ(size, arg_size);
   char* str_data = new char[size];
   Dart_Handle result =
       Dart_MakeExternalString(str,