Canonicalize uris in C++ instead of Dart for the standalone embedder.

runtime/vm/zone.h

Index: runtime/vm/zone.h
diff --git a/runtime/vm/zone.h b/runtime/vm/zone.h
index dc7b25c0b3bf054374aed8b8098aa687e1e7e220..d8ecef561aab3ad0a29d070c509a1969e7de7772 100644
--- a/runtime/vm/zone.h
+++ b/runtime/vm/zone.h
@@ -45,6 +45,10 @@ class Zone {
   // Make a copy of the string in the zone allocated area.
   char* MakeCopyOfString(const char* str);
 
+  // Make a copy of the first n characters of a string in the zone
+  // allocated area.
+  char* MakeCopyOfStringN(const char* str, intptr_t len);
+
   // Concatenate strings |a| and |b|. |a| may be NULL. If |a| is not NULL,
   // |join| will be inserted between |a| and |b|.
   char* ConcatStrings(const char* a, const char* b, char join = ',');