Restructure the Kernel string table.

runtime/vm/kernel.h

Index: runtime/vm/kernel.h
diff --git a/runtime/vm/kernel.h b/runtime/vm/kernel.h
index 81e5dcc1bc76c6a4d1499623034de9733f98ef66..f8568ee5b44eabc076deee6b90ba409ee9a848c7 100644
--- a/runtime/vm/kernel.h
+++ b/runtime/vm/kernel.h
@@ -257,9 +257,16 @@ class Tuple {
 
 class String {
  public:
+  // Read a string reference, which is an index into the string table.
   static String* ReadFrom(Reader* reader);
+
+  // Read a string implementation, which is a size followed by a UTF-8
+  // encoded string.
   static String* ReadFromImpl(Reader* reader);
 
+  // Read a string implementation given its size.
+  static String* ReadRaw(Reader* reader, intptr_t size);
+
   String(const uint8_t* utf8, int length) {
     buffer_ = new uint8_t[length];
     size_ = length;